So ... if I had $5 for every time someone misinterpreted the notion of "internal forum" and "Who am I to judge?" ... I could retire.
We absolutely CAN judge a person's motives, to a certain extent, from information available in the external forum, or at least arrive a moral certainty about them. That's NOT what "internal forum" means. Internal Forum, which even the Church does not judge, refers to the DEGREE OF CULPABILITY (OR MERIT) THAT A PERSON HAS IN THE EYES OF GOD.
I see someone holding a man at gunpoint and demanding that he turn over his wallet.
I can judge with moral certainty that his motive was to steal the man's wallet. Now, PERHAPS I could be deceived ... but then with just a bit of additional investigation, all done in the external forum, I could interview the victim and indeed determine that he was robbed of his wallet by a man who INTENDED to steal it from him, and that they weren't, for instance, acting for some TV show, etc.
At that point, I could make a judgment that he wanted the man's money or credit cards.
Now, that COULD BE wrong, but it's a very likely explanation for events witnessed. It COULD BE that he was after the man's Driver License, to do something else, like steal his identity.
Now, the next level of "internal" momtivation that can be investigated and discovered, all external forum is ... WHY did the man steal the money? Perhaps he was greedy and wanted to buy something? Perhaps he was a drug addict who needed a drug fix? Perhaps he had hungry / starving children at home. That too can be ascertained in the external forum, and let's say the guy looked high as a kite when he pulled off the robbery, it would not be judging to speculate that he likely was desperate for a drug fix.
But none of this is the line where we cannot judge.
What not being able to judge INTERNAL FORUM has only to do with the degree of guilt (or merit) that someone has in the eyes of God based on an almost infinite number of considerations that only God is capable of knowing and making, where even individuals themselves cannot judge their own internal forum. Maybe this guy grew up in a broken home and was never taught that theft was bad, but that rich people who didn't share with the poor were bad. Perhaps he had diminished culpability due to being driven by drug addiction. THAT is what's meant by INTERNAL FORUM, their assessment of the guilt, their degree of guilty, or their lack thereof.
YOU CAN JUDGE motivation in the external forum. If someone catches a man in bed with some woman other than his wife, you can safely judge that he was motivated by lust. COULD you be wrong, absolutely speaking? Well, I guess, but you're not precluded from speculating about it. I mean, with the same odds as winning the huge lottery jackpot, it could be that the woman held a gun to his head and forced him to do what he did. Or, I guess that she could be blackmailing him. But those types of explanations are relatively rare, and you're not required to withold a judgment that the guy was motivated by lust, AND with just a tiny bit more work in the external forum, you can rule out the outlier allegations, but you are not required to rule out the outlier possibilities to avoid "judging".
As for accusing Matt of grifting, that can be pieced together easily to the point of being almost morally certainty by simply observing his behavior, his language, his posting. Grifting, of course, simply means that you're motivated in large part by making money, and that this motivation will "influence" what they say or do, and you can make this judgment by, oh, who they cancel and why (+Vigano), what he allows people to say or not say, on his forum, what he himself will say and not say, where he contradicts himself, or witholds obvious conclusions that no one can really deny, just because of the impact of his audience. When someone makes his entire living off of their "influencer" role, there's an inherent "conflict of interest", where there's a tension between their need to make a living and their desire to do the will of God and to speak the truth.
But, what you're really attempting to determine is whether one commits calumny by speculating about an individual's motivatins out loud or in public.
No. Why? Simply because everybody KNOWS that you're speculating and that you don't have some "hidden knowledge" that you're revealing.
So, for instance ... if I see some guy acting effeminate, with limp wrist, lisping, you know, showing all the textbook signs of sodomitical inclinations ... is it detraction to wonder out loud whether he's at least inclined toward sodomy? No, since pretty much everyone is asking the same question, and it would require some mental illness to pretend that you don't see or notice it, and you are not required to NOT notice it. Sodomitical inclination, of course, is a special case anyway, since any right the individual might have to his good name is offset by the potential risk that he might pose to others, and so people should be aware of it, and it's not wrong to even shout it from the roof tops, so that some naive or unsuspecting individual doesn't let this guy take their son camping or the like. You can judge when behavior suggests a risk, or a possible problem.
Public interest and potential harm to others can outweigh a person's right to a good name.
AND ... Michael Matt checks that box also. See, most of us know that Matt does a great deal of harm to the Catholic faith, where he's encouraging people to recognize the Conciliar Church as the Catholic Church, and he's made outrageously heretical assertions like "the Catholic Church now endorses sodomy", etc. In addition, he prevents people who might otherwise flee that Whore of Babylon and got to a Traditional chapel by defending it and continuing to recognize it as Catholic. Matt is sitting there in the pot of hot water that being brought to a boil, and when other people say, "man, it's getting hot in here, I think I'll get out now," Matt will chime in with "nah, it's good for your health, as the heat just opens your pores", or to others standing outside, Matt say, "come on in, the water's fine". Matt is doing grave damage to souls, so we almost have a duty to call out his duplicity, his grifting, his errors.
If I were alive at the time of Martin Luther and caught him in a secret act of sodomy, for instance, I would not only be permitted but would be obligate it to broadcast it far and wide and tell everyone I knew that Luther's a perverted deviant. Why? To DISCREDIT HIM, since he was destroying souls.
If you understand that the sin of calumny is and detraction are based on a person's RIGHT TO A GOOD NAME, that's the principle, then Martin Luther would use that good name or good reputation to LEAD SOULS TO HELL, then he's LOST his right to that good name. Period. It's like in the US you have a right to bear arms, but if I know that some guy is going to go murder someone, I could confiscate and steal his gun, so violating not only his "Constitutional Right" but also his right to own his own property.
Between Matt being a public figure, an influencer and one who's doing damage, not only is it PERMITTED but even OBLIGATED for us to speculate far and wide about any impure and dishonest motives that might drive him, in order to offset he harm he's doing.
Now, if I knew someone who had defects or sins who did NOT pose any harm to others, THEN I have no reason to and on right to judge and speculate, and especially if he's a private person, since at that point you're engaging in what's known as Gossip.
So your allegations of impropriety fail every single test related to detraction and calumny. In fact, I submit that by defending Michael Matt, you could be doing harm.
Let's say, again, that there's some priest who's effeminate, limp wristed, lisping ... someone highly suspect of having inclinations against nature, if you were to try to shout down and silence people who were saying ... "watch out for that guy, since I think he's a fag", if on account of your efforts people let their guard down and then he assaulted one of their young boys, then YOU are the one who will be judged for aiding and abetting his crimes.