I think in this context (and usually) hipster means "someone who pretends to like something new or unusual in order to broadcast their superiority." That's why everyone thinks someone else is a hipster -- it's a claim about sincerity. As in "I've always enjoyed Pabst Blue Ribbon because I like the taste, but that guy over there is just a hipster."

That's also why it's a pretty jerk thing to call someone on the internet. Hipster-slingers: you have no frickin idea why people like the things they like. But try assuming it's because they actually like them. It makes the conversation a lot more fun.