19. That he himself may be judged by no one
Dictatus Papae is a well known document, but what is not so familiar is that shortly after the promulgation of Dictatus Papae another syllabus was published. Called Propriae auctoritates apostolicae sedis ("The Powers Proper to the Holy See"), this document was meant to elaborate on the theses of Dictatus Papae . Issued sometime between 1075 and 1085, these theses should be read in conjunction with Dicatus Papae, which it is meant to support and expand upon.
Thus, we see that thesis 19 of Dictatus Papae is expanded in Propriae auctoritates apostolicae sedis, the seventh thesis of which says:
7. The pope may be judged by no one, even if he should deny the faith, as is seen from [Pope] Marcellinus
I am not competent to comment on the authority or theological import of the document, but future discussions about theoretically deposing a pope should most certainly factor in this document, as it was promulgated under the Pope Gregory VII specifically in part to address this very question.