Except that what a lot of startups mean by "fit" is "they think just like me" or even, "they have a similar background to the rest of the team." Sure, that may make them easier for you to talk to or hang out with, but it reduces the diversity of the team, and increases the likelihood of groupthink. The other things you mention (after "fit in your culture") are not what most people mean by "fit". "Fit" is an intangible that can be used by screeners to keep from going out of their comfort zone.