In most instances, “authoritarianism” is a more rigidly defined term than simply meaning “exercises authority.”
E.g. Wikipedia defines it as
a political system characterized by the rejection of political plurality, the use of strong central power to preserve the political status quo, and reductions in democracy, separation of powers, civil liberties, and the rule of law.



Picking apart the single definition used by one entity doesn’t mean the term itself is completely meaningless.
But fine, I’ll bite, just for fun:
That’s “whataboutism.” Or alternatively, it’s “authoritarian realism”—a term I just made up which refers to any view that assumes a nation has to centralize powers to exist because that’s how the world under capitalism currently operates.
So 1. You just gave a counterexample to your first point, and 2. I guess the metric depends on who you ask. It could be reductions from a historical state (as we could say of e.g. the current USA compared to North America’s political systems prior to european colonization), or compared to some standard of liberty (e.g. your use of USSR).
I can agree with your first point and still posit that the term is meaningful: e.g. authoritarianism isn’t a binary state of extistence, but rather a spectrum that different states can be compared on; all states can be authoritarian to some degree, but some states are more or less authoritarian than others.
Or to put it another way, saying “authoritarianism” is meaningless because all states exercise authority is like saying “conservativism” is meaningless because all living creatures seek to conserve resources (to some degree).
I agree that language is an imperfect map for the real world we inhabit—and I especially agree that the language (as with any social tool) gets abused to manipulate people—but I don’t agree that those facts make the terms completely useless in communication.