I think you are onto something, but it is not accurate to say that libertarianism and anarchism can not be the same thing. Big “L” political “Libertarianism,” as in the “Libertarian Party” is decidedly not anarchism, I agree.
That said, “libertarian” can and is also defined simply as a person who “believes in free will.” That’s just a semantic issue though.
As for Voluntaryism/Anarcho-Capitalism, why do you claim these are not anarchism? The capitalism referenced does not indicate crony capitalism/statist capitalism, but simply free commerce. Anarchy literally just means “without rulers,” i.e. those who control via the initiation of force.
My understanding is that anarcho-capitalists and small l libertarians both want a state to protect their markets and liberty - if you want a state, you are not an anarchist. In this case, by state, I mean a centralized control system bigger than a community or city, i.e. a multi-community central power embued with the ability to utilize force
Also, and probably more importantly, capitalism itself relies on exploitation of workers for the accumulation of wealth and power - both of which fly in the face of anarchism's core tenets. IMHO.