Towards the end of the war there clearly wasn't US support for the USSR; we were racing them towards Berlin (and lost) and raced them to get Japan to surrender (won that one). The optics of completely abandoning someone you are ostensible allied with are poor.

All of that is orthogonal to the fact that over a third of the people who died in WW2 were Soviets, and many people do not realize it, which is the point the grandparent comment was making.