Trump pretty much did what he always did, which is read the room and react in the most obvious way in the moment. People like Mamdani, better to embrace him and try to catch some of that wave than remain opposed to him. This is all part of capitalist co-opting of reformist blah blah you know that part.
idk what's going on in Mamdani's head. He probably sees being gracious to the president as a better path than being oppositional to him. But we simply won't know his true colors until he's in power and ICE raids are happening in New York and we see his response. Power doesn't corrupt, it reveals.
He probably took a plane over a train for the most boring reason imaginable, like the White House wanted to set the meeting at a specific time and that was the only way to get there fast enough. Or maybe the WH sent him a plane ticket in some kind of crypto anti trains power play.