Fr. P pulled this straight out of the Novus Ordo playbook. I wouldn't expect an explanation for the "separation" from Ambrose. This is Pfeiffer's Summorum Pontificuм. A docuмent which is ideally meant to reinforce a status quo (of Catholicism) that no one paying attention believed still existed, but a docuмent which satisfies those who either aren't paying attention, or are, and are just willfully ignorant of the situation.
Besides, he can't give an explanation without looking incompetent (at best). What's he going to say?
"After much deliberation, we've concluded that the man to whom we presented you as a Catholic priest and bishop, is not."
There is literally no explanation that allows Pfeiffer to save face. Ergo, he won't posit one.
Still not entirely sure how he ever thought he'd be able to pull this one over on anyone. It's no secret that he's a revolutionary with an ego, but even at that, did he really expect to be able to keep a sustainable following after inviting Ambrose?
Keep in mind that discerning Catholics have left their place of worship over far more innocent errors than this.