I don't even think it's that. It simply isn't possible to keep the US on their side while the country is being run by Russian lap dogs. What he can do is play along so there isn't anything to support Trump's bullshit attempts to paint Zelenskyy as a warmongering dictator who doesn't want a peaceful resolution.

It's not going to stop Trump from wanting to join the war on the side of his Daddy Vladdy, but playing ball makes it harder to justify doing so.