"Spurious" is a generous word for this contention.

In fact, the idea that Jews promoted Marxism postwar via the Frankfurt School and other academic perches is bullshit.

This. Idea. Is. 100%. Nonsense.

For one thing, there are many shades of Marxism and these conspiracy theories treat Marxism as an undifferentiated mass unified in thought and action. This would come as a shock to anyone who actually knows Marxists in the real world in which there is often intellectual Balkanization and a large number of very sharp disagreements.

Second, the idea that the Jews dominated Marxism is an old one, but one not substantiated by the facts. Yes, there have been more than a few Marxist Jews. But they have always been outnumbered by their non-Jewish compatriots. This online canard traces itself back to fascism and the far right jeremiads claiming that Soviet communism was a plot by nefarious Jews to destroy the West.

/u/commiespaceinvader and I have a much larger answer that delves into the idiocy of these conspiracies as well as digressions on the politics of word fonts.