I'd argue that the solution proposed by Ethereum in this blog post is not antithetical to mainstream Libertarianism. It actually fits perfectly well into the role the majority of libertarians believe a state should take.
To begin with, from my understanding they merely proposed a solution which the community has to agree to implement. Just like modifying the bitcoin codebase.
It's still ultimately an additional layer of decentralization in between. Taken as a whole - even if Ethereum takes action against the attacker - what DAO represents would still be very very far from the representative democracy style system that libertarians take issue with.
Importantly, libertarians are not all anarchists (or 'crypto-anarchists' or 'anarcho-capitalists' to be more accurate) who believe in total decentralized control structures. Mainstream libertarians wish for a minimal state or "night-watchman" state, a not the total absence of a state.
This the most prevalent myth about libertarianism and the faulty premise of most attacks against it.
Even many hardcore anarcho-capitalists are against the idea of decentralized judicial and law enforcement systems - as they see it as unworkable.
In the book "Anarchy, State, and Utopia"  popular libertarian thinker Robert Nozick argues that
[..] only a minimal state "limited to the narrow functions of protection against force, theft, fraud, enforcement of contracts, and so on" could be justified without violating people's rights.