I don't think it's fair to say that "anti-imperialism" was one of the requirements when many of the western ML/Stalinist groups that would generally fall under any anarchist's definition of "tankie" were very much chauvinist/nationalistic. Like that describes basically nearly every "communist party" in the west.
Obviously the modern anarchist usage of the term was fairly different to the origin of the term anyway, which meant a self-proclaimed communist who supported Soviet imperialism in Hungary, which Stalinists were/are opposed to. But I guess they re-used the term now that Khrushchevites are not really a thing anymore.