What do you make of Brother Peter Dimond and the sedevacantist claim that the pope is not the pope?

He is an intelligent man who surely has good things to say, but he has used his free will to declare that the pope is not the pope, making him a sedevacantist. He has assembled his own interpretations of various Catholic documents and concluded that once a pope commits what he judges to be an unrepented heresy, the pope ceases to be pope. That is not true, thank God, because otherwise we would have had this scenario dozens of times in Church history. The only one who can declare that a pope was not truly a pope, that is, an antipope, is a subsequent pope, because only he has the authority; in Dimond's scheme there is no pope after the pope to make that declaration. Brother Peter has no authority whatsoever, so he can voice his opinion but it moves nothing. And note that no pope has ever issued dogmatic rules for determining whether a sitting pope is valid; all we have are the opinions of medieval theologians like Bellarmine or Suarez, and when something this serious is at stake, opinions without authority behind them amount to nothing. No layman and no cleric, however holy he seems, has been given power to declare the sitting pope not the pope; only Jesus Christ, who put him there, can judge him.