Pontius: counterfactuals are not all equal in nature. This one's a complete piece of crap. For further reading, our own OP, and the old fart whom he cites as a "source" ought to examine a well constructed example of how countfactuals work, or don't, in a book written by Niall Ferguson called Virtual History.
"Truman as unknown" ignores that Truman didn't act in a vacuum. He also learned on the job. See Berlin Airlift, 1947, in which my father was also present.
As to "isolationism:" the Draft was reinstated in 1940, and our people of political power (the blue bloods on the East Coast, where a lot of political power remained in the post WW II era) were supporting the Brits via lend lease well before Pearl Harbor. "Isolationism" was a sentiment that killed the League of Nations (post WW I) but that isn't all that did it. There was other internal political bickering between the Executive and Legislative branches involved in that showdown over a treaty. "Isolationism" is hardly the force in mid century that people like to pretend it is once we went large. Our commitment in WW II was orders of magnitude larger than our commitment in WW I.
Factor also ignored: the Red Scare was alive and well in the 20's, 30's, and 40's which would inform any American position in mid century. Harry Truman was not tone deaf. The Red Scare had not gone away, see the Alger Hiss case a few years later.
Russia versus UK in 1946 does not just involve those two nations. It becomes the concern of anyone and everyone in the neighborhood. Left unsaid: who does the US supply? The UK? USSR? Both? Neither?
As above stated: the counterfactual offered isn't worth the paper it wasn't written on.