I'm of two minds on the matter. On one hand, one can very seriously argue that fascism and authoritarian 'left' groups are distinct in their proclaimed thought processes. Fascists very much hearken to the idea of an eternal conflict and a single national leader, while authoritarian leftists, in theory, are seeking an actual end goal of a stable society without a strongman.

On the other hand, in practice, there's very little difference not only in policy, but also little difference in justification by actual pracititioners. Tankies go all-in for the same cultural chauvinist and hegemonic arguments that fascists do, they just call it 'anti-imperialism' instead of 'national vitality' or whatever the newspeak neonazi euphemism of the day is. Tankies proclaim that they aren't in it for eternal conflict, and then break out the death-cult-of-heroism eternal ultra-martyrdom common to fascists and religious fanatics anyway. Tankies talk a big game about making a united front, but then immediately shut down all opposition, no matter how minor the disagreement, as 'reactionary' and call for them to be shipped off to re-education camps.

Insofar as there is a difference, it's like paint of chartruese and bile-green. Side by side you might be able to differentiate them, but seeing either one spilled onto a perfectly good table, you probably aren't going to care all that much about the distinction; they're both pretty vile, and both in nearly the exact same way. In that vein, I prefer to emphasize that tankies and fascists are really not that differently, fundamentally and practically, than to emphasize the minutiae of theory that differentiates them.