I’m kind of confused here. Contracts are enforced by the government, right? Requiring a piece of paper that the government supports in order to obtain employment does not seem very libertarian.
I’m just playing devil’s advocate here, I don’t actually care about libertarianism because in the US it’s a label self-applied by many kooks (for example, just look up the Libertarian Party nominees for president in… any of our elections)
By US standards, in the US job market, “unions are a libertarian concept” is something that only someone unfamiliar with US labor dynamics and the local meaning of “libertarian” would say. It’s a pointless exercise of “Look how I define this word outside of meaningful context!”