Anti-imperialism had a history of being used (successfully or not) by oppressive, expansionist states to enlist the aid of or to commit imperialism against colonized countries. The Nazis attempted the former in, especially, the middle east, and Soviets did a fair bit of the latter. IIRC the Kaiser's government made some similar moves in WWI (but every major player in that war was imperialist, more or less). Anti-imperial revolutions or movements sometimes really were Soviet attempts to install autocratic puppet governments.

On the flip side, of course, so were many US and NATO interventions—but this is why "anti-imperialist" might be taken as possibly actually meaning anti-NATO and pro-Soviet—much "anti-imperialism" was really just a smokescreen for Soviet maneuvering, same as "liberation" et c. often has been, when the US does it—it'd be foolish for actors on the international stage to take either claim at face value, because they've a history of being half-true or not at all true, just a PR cover for realpolitik Great Games.

Hell, even the British, one of the most-colonial of colonial powers ever, used this against the Ottomans, and it was a lie then, too. Seen Lawrence of Arabia? Stirring up Arab nationalism against the colonizing Turks, not actually for their sake, but just as a convenient play in a larger game?

This is why US elites at the time might genuinely have believed that going to war in Vietnam was a good idea—even bordering on necessary—and perhaps, truly, believed it was morally defensible.

The US has pretty good relations with Vietnam now