NEW - Biden says "The president wrote a very generous letter," referring to Trump. "Because it was private, I won't talk about it until I talk to him. But it was generous."
This taken from the US Military Voice Telegram Channel.
I'm betting the 'generosity' was the deal Biden cut with Trump...
Generous could be used differently.
In a cooking recipe, "use generous amount of salt" means a good amount.
Maybe the letter contained tons of insults, then it is generous in it's amount of insults.