Libertarianism is a sliding scale. Many libertarians are ok with some government regulation and things like anti-monopoly laws. Others believe they aren't necessary and that most monopolies are created because government regulation rather than in spite of it.
I have never, heard any libertarian publication in the internet advocate for anti-monopoly laws. And the second position is fantasy, because every single economic theory (including those by Von Mises, Friedman and other "heroes" of libertarians) can describe the existence of natural monopolies quite well.
Doesn't take a genius to know that these extremist positions are the result of ignorance in the way money and power flow through society. Unfortunately, it's the sort of ignorance that reaches into the "not even wrong" territory.
There's no need to advocate for anti-monopoly laws because they already exist. They just don't spend time advocating against them.
Natural monopolies are a different issue (anti-trust laws can't affect a true natural monopoly because it's by definition impossible to break up.) Some libertarians dispute that they are an issue at all, more on that position here: https://mises.org/daily/5266/ Others believe the government should handle things like public utilities, just nothing else.
