Libertarianism is properly a socialist philosophy, but it's been coopted by the far right in America who then started exporting their bullshit to the rest of the world. It's entirely possible for two people to call themselves "libertarian" and have next to nothing in common in their understanding of what that means.
Most libertarian support of unions in America is a bad joke. It's meaningless feel-good rhetoric that completely ignores the entire history of unions in America. Corporations have a million ways to crush unions and union organizers. Without government regulations requiring corporations to engage with unions, unions are next to impossible in reality. But that's just fine with the kind of people attracted to the Libertarian party, because they don't live in reality.
Excerpts from the Wikipedia entry on libertarianism:
And
Don't feel too bad. Having no fucking idea what you are talking about just makes you a typical American style libertarian.
As for centralizing power, corporate personhood and broad deregulation are about the most radical systems for centralizing power that have ever existed. You are still ignoring the entire history of conflict between unions and corporations. Unions had their day using the "libertarian" model and all that came from it was disaster. It wasn't until the labor movement gained political power and had pro-union regulations put in place that unions had any real ability to negotiate with corporate power. But that's all reality so it's irrelevant I guess.