Anarchism is a leftist theory of everything because it is radical. It's easy to just be a socialist, and to not be a feminist or a queer ally or a supporter of indigenous sovereignty. And it's easy to be a feminist without being a socialist, queer ally, or supporter of indigenous sovereignty. And all the other permutations too. But anarchism, as a political lens, points us towards all of these different struggles and says they are the same. I am not wholly convinced anarchism is the only leftist theory of everything, but I have yet to see an alternative. Rather, I see dozens of alternatives within anarchism, such as soulism, transhumanism, mutualism, and so forth.

An anarchist has solidarity with all of the leftist movements, works together with all of them. If one wishes to run any sort of leftist political movement while maintaining alliance with the entirety of the left, I believe explicitly endorsing anarchism is a good way to get that done, from a practical point of view. After all, the nazis went after the communists before the socialists, because communism is stateless and is therefore aligned with anarchism. The nazis knew that the non-communist socialists, who support the existence of the state, would not offer as much resistance as the communists to the attacking of other leftist groups.