Mr. G, on the other thread, you wrote that Sean stated the above definition of "sedevacantist". Whereas it is a more precise definition than the one provided by 2Vermont, it is still lacking because it does not provide the cause of one being a "sedevacantist". The Sedevacantist groups would see this definition as being incomplete as well.
No, every sedevacantist is defined by this alone:
The man claiming to be pope is an antipope, and there are no other claimants who are pope.
Why one believes this to be the case is irrelevant to the fact that it is believed.