All in all, "Tankie" is just the modern version of "Pinko!" I've seen Anarchists called Tankies, every manner of Marxists, non-Marxist Socialists, and more. It truly lost all of its meaning and is just Lemmy's Red Scare.
A huge portion of that comes from Lemmy.world, which is the largest instance to defederate intentionally from Hexbear and Grad, which has the presumably unintended effect of making Zionists and other far-right wingers feel comfortable in an Anticommunist instance.