I mean, if you want a real genuine answer, it's that simplifying the entirety of political thought into a binary is a rediculous premise to begin with, and highlights one of the core fallacies that the human condition leads to time and time again: that of false dichotomy. Calling the political spectrum a circle is exactly as absurd as calling it a line, and taking either of these paradigms to be literal and infallible is to grossly misunderstand politics.
My point is more that both routes, left or right, have a path through extremism into authoritarianism. Try not to take the silly analogy I used to communicate this point so literally.
I do believe that extremism lends itself to authoritarianism. The deeper you are rooted into your belief structure, the more likely you are to believe everyone else has gotten it wrong, and the more likely you are to think imposing your beliefs on others is in their best interests. The circular model that I proposed is simply a way of highlighting this.
While I am sure this isn't true of all libertarians, they tend to be ogliarchs (or wannabe ogliarchs) in sheeps clothing. We may have another word for rule by the rich and economically powerful, but I do not think the gap between them and fascists is wide enough to avoid the blanket of "authoritarianism." I do think libertarianism is an extremist idea that just leads to a different flavour of authoritarianism, thus my point.