By saying 'not a good fit' requires additional details to be valid you are actually acknowledging that 'not a good fit' is not a valid reason to not hire someone, those additional details are the valid reason to not hire someone.
When people say "not a good fit" they mean "cultural fit" ie, not related to technical skill, which is what the context of the parent post was implying. Maybe that's not so ubiquitous, but I've come across a technical person not be able to point out exactly what they don't like about another technical person's skills.