This "race is a social construct" meme is something I have been trying to grapple recently.
I think the point is that races are based on a handful of cherry-picked phenotype differences. On the one hand, these characteristics are a decent proxy for human migrations in the past and might map to evolutionary events with clinical implications. On the other hand, another set of genetically conditioned characteristics would be similarly good at that. In an alternate universe, humans might have defined race based on hair color and height, and that would possibly match evolutionary history in another useful way.
EDIT: correct link below.