This is a huge problem for Siscoe and Salza and their followers.
What happens if a group of bishops intends to convene a General Council, and the heretic Pope instantly deposes every single one of the bishops who has agreed to hold such a Council?
You could argue, "oh, well, that would be unjust." No, sorry, there's nothing "unjust" about the Pope replacing a bishop. He's not excommunicating them, just replacing them. He could replace a bishop because he doesn't like his body odor, for any reason, or for no reason, since they serve entirely at his pleasure. If, as per Siscoe and Salza, this man continues to hold authority, then the bishops' authority derives entirely from his appointment of them ... and he has ever right and ability to replace them, for any reason.
This is an absolute check-mate against Siscoe and Salza.