Thing is left right isn't much of a line, no matter what shape it is.

The right is a point, they'll get behind a populist and go to jail for them.

The left is a scattered mess of disagreeing elements who hate each other almost as much as they hate the far right.

And of course they both hate the dreaded "liberals".

Horseshoe theory is dumb, but it's really just an observation of the loudest ideologies on the far left and far right, which both happen to be authoritarian. Authoritarianism becomes necessary as you move toward the extremes because you have to coerce some people/classes to accept the system. And it's true that real-world instances of both Fascism and Communism have been authoritarian, and so they share some things in common. It isn't a particularly nuanced or deep understanding, but it is true that authoritarian forms of gov't are authoritarian. The difference lies in the details. Communists used authoritarianism against capitalists and the nobility, and fascists used it against minorities. Horseshoe theory conflates "authoritarianism" with extreme Left and Right-wing ideologies. This contrasts against anarchism (and by extension the broad anti fascist movement), of course, which is extremely anti-authoritarian (hence why horseshoe theory completely falls apart here).