The main defense for this accusation, from the R&R "authorities" is that the Conciliar Popes have not used their teaching authority for real. They are using a kind of "dialogue mode" Papacy, on which they merely propose ideas, and don't really teach.
Right, as if the V2 papal claimants were merely engaging in speculation with V2 and considered it entirely optional ... which is why they persecuted Archbishop Lefebvre and the SSPX for decades, and now Bergoglio is claiming that rejecting the teaching of V2 constitutes schism (one of the charges against +Vigano).
In any case, very few modern R&R know that +Lefebvre himself did not deny the basic Catholic teaching that the Papacy is guided by the Holy Ghost from destroying the Church in the manner we have seen. He merely prescinded from the SV conclusion because he couldn't commit with the sufficient certainty regarding how this could have happened. He did not deny the MAJOR of the SV position, as most modern R&R do (with the exception of Father Chazal, if you accept his self-characterization or self-identification as being R&R), simply felt that the MINOR, namely that these men were non-popes was not sufficiently certain for him to commit to it. He never ruled it out, and in fact considered it to be quite possible ... just never committed to it.
If that's the kind of thinking behind an R&R position, there's nothing un-Catholic about it whatsoever. But it's in rejecting the MAJOR that one runs afoul of Traditional Catholic doctrine.
Unfortunately, +Lefebvre was not emphatic enough and didn't repeat this distinction often enough, so that much of the modern R&R movement have morphed into the Old Catholic mentality and Old Catholic doctrine of the papacy.
If someone wanted to say that these men have been legitimate popes but were being blackmailed (not acting freely) or even that they have been replaced by doubles and tied up in a dungeon somewhere, while I wouldn't agree, I'd have no problem with the position from a doctrinal standpoint. More power to you. I also have no problem whatsoever with Father Chazal's position. Nor do I have any issues with Archbishop Lefebvre's position (except for when he was cavorting with the Modernists in the early 1980s in the manner of +Fellay today). What I have issues with are the many R&R today who are basically Old Catholics.