That isn't true though: the Nordic countries are undeniably authoritarian from a leftist perspective, but you never see Nordic model socdems being called "tankies". Failed leftist projects like Catalonia or the Paris Commune were also undeniably "authoritarian" by the definition applied to more successful projects, but supporters of them are never called tankies. The Black Panthers, Nelson Mandela, Martin Luther King, all supported "authoritarian regimes"

What it really boils down to is serious opposition to Western liberalism.