He’s not. Trump is old and is feeling the call of the darkness at this point (I’m sure Putin can relate). Trump is looking for legacy. Getting him on board with bullshit like this is designed to nudge him towards being more on Russia’s side, namely when it comes to sanctions and aid for Ukraine.

unless he’s doing it to make the US spend money on nonsense.

Nah, these are loyalty tests by dictators, to prove that they will follow them no matter how crazy the idea is.