> it almost always comes down to a politically correct version of "we didn't like hanging out with him/her"
Startups run on morale. This is a pretty important feature of a startup employee. Even if that's what "culture fit" means that doesn't invalidate it as a reason to fire.
Startups live or die based on their ability to quickly deliver product, and morale alone won't get your product delivered. An employee's ability to deliver results is more important than their desire to have a beer with his coworkers. I'd much sooner hire someone with competence and experience than someone who was fun to work with.
Startups run on morale. This is a pretty important feature of a startup employee. Even if that's what "culture fit" means that doesn't invalidate it as a reason to fire.