The position of Archbishop Lefebvre in regards to whether a putative pope is truly pope was based on the evidence of the putative pope's public manifest formal heresy.
But what about the Pope’s papal commands to abandon the Traditional rite of the Latin Mass? Are we not bound to obey him? No, we are not bound to obey him because it is not a lawful command, as Archbishop Lefebvre always said, and as the Catholic Church has always said. The Pope has no power from God to command just anything that comes into his head.
As is said in the OP, this ^^ is what it's always been all about and has nothing whatsoever to do with a "putative pope" or evidence of heresy, or anything whatsoever related to sedeism. +ABL tolerated sedeism as much as he could, but never agreed with it, preached against it and never bought into it.