Is that discrimination? Somebody can en an annoying prick regardless of their background.

If the candidate is a protected class and they are rejected for "cultural fit" it will be an easy case for EEOC to raise a discrimination case.

This is effectively how Harvard was rejecting Asian applicants. They created a "personal fit" / cultural fit quality that Asians scored low on . Supreme Court found this to be discrimination.

It doesn't matter if you are truly discriminating, it matters how well you have tangible evidence of the employee not meeting the qualifications for the role.