But most racial classifications would not separate the Finns and the French. Sami Finns, maybe. But the difference is there for non-Sami Finns as well. And few would say that the Normans and Celts were racially distinct. Ethnically, yes. When almost any population stays isolated long enough, they develop tendencies. If this is what you classify as race, then the Travellers fall into it. They even have the tendency to develop certain diseases which are rare in the general population.
But when someone calls race a social construct, they are saying that the idea that someone like my half-brother who looks black should have more in common genetically, even without going on to get into the really racist bits, with a random black guy than me, his white half-sister is not scientific or relevant. It's ridiculous and tribal. The really racist bits are that his potential or characteristics are different simply because he inherited his fathers more obvious skin colouring (and likely other physical differences that would classically be grouped with "race"). It's not denying that there are genetic differences in populations. It's denying that the differences are as important as we make them out to be. It's how we have handled these differences when we encounter them which is important and makes it a social construct.
And yes, my birth family was basically like a miniature version of the UN.