Buuuuut, they bought it with fiat currencies, in many cases USD. So... right there the entire idea breaks down. Still, maybe that's unfair. A valid criticism of this argument is that it's a variant of the the serf comic argument (or genetic fallacy) in which those embedded in a bad system may not question it because they benefit from it.
The real problem from a libertarian perspective is that bitcoin (and really all PoW blockcahins) just encodes how a new State, a plutocratic one based around processing power and energy availability, is formed. This is in a very literal sense what bitcoin encodes, with the miners being the only folks with any real votes (in that if miners as a group refused to mine other currencies and actively discourage others from doing so, other currencies cannot scale and are locked out).
Not that bitcoin is scaling all that well. Or eth for that matter.
As we've seen from recent research, Bitcoin is slightly better than existing systems at discouraging monopolies (because even accidental misuse could crash the currency), but I seriously doubt a mining duopoly dictating most of the future of bitcoin is something that any libertarian actually calls a positive outcome, other than a snarky nod to damaging existing states.
The long term evolution of the system is pivotal on some new, non-PoW scheme. PoS is the major contender right now. There might be other methods as derivations (e.g., PoW coupled with physical location, PoS with term limits, PoS with accreditation, etc). Non-PoW systems will also scale much more effectively than PoW systems, as well.
