As I said, I don't know what those fellows did in a political sense, what compromises they might have made w/respect to their ideas. And apparently the author didn't know either. Instead there is mostly guilt by association.
Now it could be that in practice anti-state always devolves to state authoritarianism, an interesting and clearly fatal flaw in the idea. But the article is superficial respecting the ideas and lacks any more specific political information than the fact that a Chilean economic institute invited some big names down and published them. For all we know Buchanan (who's one of the least arrogant, big-shot economists I've met) wrote in an effort to moderate the machinations of the emerging Chilean authoritarian state. The public choice lit is pretty ecumenical in its condemnation of private sector combinations' use of the state.
You can criticize anarchists for having ideas that lead to the negation of anarchism, but you can't criticize anarchists for not being anarchists.
mbs