Probably the real reason is if Biden gets the nomination, then the REAL investigations into his corruption start and that will implicate Obama. Obama wants to save his own miserable hide, so he doesn’t want Biden to be nominated. Ergo, he won’t endorse Biden.
Maybe that is why they (the deep state, whistleblower, Shiff, Pelosi...) were so panicked when Guiliani started poking around in Ukraine (on behalf of Trump). He was or is close. Wonder how many Democrats (and Republicans) would be implicated if dirt is discovered? Sounds like all of the major reps in the House and possibly Senate. That would be a major upset if half of Congress had to resign.