Not a bug. 100% true. But don't get your hopes up that SSPX and Resistance types will accept it.

I think you might be conflating two different things: 1) the Church's position on the una cuм...

Agreed. Fr. Cekada did not invent the teaching that a priest should not pray una cuм with a heretic. That is just traditional Catholic teaching.

But Cekada did invent the teaching that Catholics, who attend Masses where the priest prays una cuм with a heretic, should be denied Holy Communion at Sede chapels. Can we agree on that?

As in so many other areas, the dogmatic non-una-cuм crowd engage in false dichotomy.

Yes, if you deliberately put an Anti-Pope into the Canon where it's tantamount to a profession of adherence to a man you know to be a non-Catholic, that's a grave thing and puts you outside the Church.  That might even invalidate the Mass actually.

But the simple material / objective act of putting the name of some guy in the Canon who happens to not be the Pope (due to your mistake in judgment) is NOT the same thing.

I argued on X with one of Bishop Sanborn's top priests about the matter, and he insisted that putting "una cuм Leo" (I pointed out that it should be "una cuм Leone") in the Canon is the same thing as saying "una cuм diabolo", where both defile the Mass.  He's basically saying there's no difference between a priest who puts Leo in there because he thinks Leo is the pope, or probably is the pope, or perhaps just might be the pope, or just because he's a "material pope", and a Satanist who professes allegiance to the devil.  This is so absurdly in violation of even basic common sense that it leads to neuroses and psychological issues where the mind becomes severed from any attachment to reality.

St. Vincent Ferrer, who just happened to get the identity of the Pope wrong, he might as well have been a Satanist, and those Masses were sacrileges, displeasing to God.  Or some guy in the Middle Ages who didn't know until a year later that the previous pope he had been putting in the Canon had died.  And on and on.

Those are all MATERIAL error.  NOBODY who puts Leo's name in the Canon does so thinking, "Yeah, I know Leo's not the Pope but I'm putting his name in there to profess my allegiance to the Conciliar Church rather than the Catholic Church."  NOBODY!

Even with his "una cuм diabolo", let's say there was a poorly educated Italian priest in the Middle Ages who after the election of a new Pope heard a priest say "Papa Diabolo", where he's saying that the newly-elected Pope is a devil, but the not-so-bright mistakenly concludes that the new Pope's NAME is "Diabolo" and offers Mass "una cuм Diabolo".  So even with that example, it's completely different than Anton Lavey getting himself ordained and offering Mass "una cuм diabolo".

FAILURE TO MAKE DISTINCTIONS ... the cause of nearly all Trad errors, whereas making the proper distinctions is key to solving them.  They talk about St. Thomas all the time, but how many have actually learned his method of thinking (his and that of the other scholastics)?

St. Vincent Ferrer, who just happened to get the identity of the Pope wrong, he might as well have been a Satanist, and those Masses were sacrileges, displeasing to God.  Or some guy in the Middle Ages who didn't know until a year later that the previous pope he had been putting in the Canon had died.  And on and on.