As I have said, picking individual outliers does not invalidate a category. I think you’ve got it backwards. We interpret racial characteristics through a social lense. But the characteristics do, themselves, exist. And they are easily grouped (not exclusively, but generally) into the categories we call “race”. And we’re not randomly picking traits. They’re inherited via a common ancestry. As you said, physical, observable traits.
Could Harris pass as Sicilian? Probably not, but even if she could, she doesn’t have any Sicilian ancestry to my knowledge, so it would be inaccurate to call her Sicilian. Or Indian or Korean or whatever. She could call herself Nordic and we would laugh at her.