All sedevacantists have a problem with that position because they believe that it cannot be reconciled with Catholic teaching. It's the reason why sedevacantists exist.
Please don't speak for "all sedevacantists" because you don't and you cannot.
While it is true that "all sedevacantists" have come to those conclusions, which is why they are sedevacantist, it is not correct to say that all sedevacantists believe that these conclusions are dogma and that failure to reach these conclusions is heresy.
Those who believe Benedict 16 is the pope of the Roman Catholic Church are, in my opinion, wrong; but they are not, by that fact, outside the Church. They are outside the Church only in as much as they subscribe to the heresies that Benedict 16 teaches.
Whether Benedict 16 is a true pope is a matter of fact. Like many facts that are ascertained by witnesses and deductive reasoning, not everyone sees the same evidence or understands the evidence they do see in the same way. Only history will tell us who is right just as history tells us that Athanasius was right while Arius was wrong or that Eusebius was right while Nestorius was wrong. At the time, many people were confused and faithful Catholics differed on each case.
The SSPX, for example, is still Catholic as they have not, as an organization, defected from the faith.
On the other hand, I believe that dogmatic anti-sedevacantism is equally as bad as dogmatic sedevacantism for anyone who is dogmatic on either side of this
prudential judgment refuses communion with those who disagree with their
personal opinion on the matter of the identity of the pope. Matthew and I disagree on the answer to that question, but neither of us declare that the other is not a Catholic for that reason.
CathInfo allows (as opposed to Angelqueen) Catholics to argue the question in a rational manner. Individuals may be convinced of the opposing arguments and reconsider and change their conclusions, but they are not "converted". (Interestingly, the SSPX frequently talks about the need to "convert Rome", but that is another topic altogether.) As long as there is a single claimant to the papacy whose claim has met the external forms (i.e., the people appointed as cardinals meet in conclave and a successor is elected and is accepted by "the world" to be the new pope) the question will remain open. The question of the "heretic pope" is not one that has clearly and unambiguously been settled.
In any event, in practice, all traditional Catholics who reject the heresies of the Conciliar Church and the docuмents that issued from Vatican 2, act in the same way toward those heresies and the papal commands and teachings that emanate from those heresies. The difference seems to me to be one of semantics; which is why, I think, dogmatism by either party could be schismatic and should not be tolerated by Catholics.
Now...the dogmatic sedevacantists have just declared me a heretic.