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.
Not that they were libertarians, but the presidents during the 1920-21 Depression acted slowly, and the economy bounced back incredibly quickly. From the article: However, by the time Harding had called his conference, the country's economy had already shown signs of rebound, and by 1922 was starting the economic boom of the 1920s and merely allowed for President Harding to claim success.