Late and gay, but this is the worst play possible with a poor hand.
If he considers himself unfit to run for re-election, and despite the terse prose in the letter, what else can be his reasoning, how can he be fit to continue to hold the office? It's only four months till the election. He can't be fit in July and unfit by November unless he's fucking dying, of which we have seen no evidence.
He needed to step down as President too, and give Kamala whatever slight benefit she got from being technically the incumbent, if it was his true intention to try to get her elected.
Was the idea of effectively imposing America's first woman, and second Black, president a bridge too far for the donors?