Bro Dimond talks as though he's the Pope.
That's because he thinks he's merely restating Catholic dogma. So, for instance, if I "dogmatically" proclaimed that there are Three Persons in one God, that is not somehow pretending I'm the pope, but merely restating what various Popes have taught. That kind of dogmatism is Catholic. So the Dimond Brothers will state "There's No Salvation Outside the Church" dogmatically, and that's no different than my proclaiming the Holy Trinity dogmatically.
But it's at the next step that they fail to see the problem. People say they hold that there's No Salvation Outside the Church, but then make various "distinctions" and "interpretations" of what that actually means. So the Dimonds claim that these distinctions are tantamount to rejecting the dogma OUTRIGHT. But they're not. It's a matter of rational human argument whether the distinction is tenable in light of the defined dogma. That's where they fail. They claim that if they can produce a syllogism demonstrating that this proposition contradicts dogma, then it's tantamount to rejecting the dogma.
They make no allowance for the fact that they could have erred in their argument. Also, even when the argument APPEARS to be valid, if one of the premises is less than
de fide, even if it's theologically certain, for example, then the conclusion of the syllogism can be no more than a theological certainty (not a heresy). That's due to the principle of logic
peiorem partem sequitur conclusio.
In fact, it is perfectly OK to ARGUE that some distinction entails objective heresy. And some day the Church might agree and confirm your statement that it's heretical. But, until then, and THIS is the exact point at which they cross the line, it is not OK to consider someone to be a formal heretic and outside the Church. You see, if someone bothers to make distinctions against defined dogma, that's prima facie evidence that they at least want to conform to Church dogma. Only God can judge their sincerity in the internal forum, and then only the Church can condemn the proposition as formal heresy in the external forum.