Truman does get credit for wading in .... but it's very much his fault that we did so at an immediate disadvantage.
I would say, too, that he badly misread the situation leading to the entry of China into the war. He had a lot of help there, of course, and it's probably the case that he could not have stopped the Chinese invasion. But we were not at all prepared for it -- in fact, Truman ignored a direct Chinese warning of intervention three months before they invaded.
That is 100% on MacArthur. I'm sure you know that he insisted, against all evidence on the ground, that the Chicoms were sitting things out north of the Yalu. Any POTUS relies on G2 from his theater commander. In any event, I rank Truman fairly high on my list of great (ok, really, really good) presidents...he is the only dem on my list.