That would actually be hilarious to think about. Biden got forced out, and he, like the other top democrats, knew Kamala would be a bad candidate. So, to get back at those who pushed him out, he endorsed Kamala almost immediately and anointed her as the nominee.
Or as this theory goes.
This was the rumor and there was reporting at the time it happened that this was exactly the case.
Probably giving Biden way too much credit. Most people knew at the time that elevating Harris to VP would mean she'd be the nominee someday, and that meant a likely future loss. Biden was just too busy virtue-signaling and checking diversity boxes to care. They're lucky things turned out as it did and that she didn't destroy the whole downballot.