I think saying that race "isn't a thing" is a bit counter-productive, because people can clearly see that different groups of people have different genetic traits, so it comes across as denying basic reality. Even in medicine, it can be useful to know someone's "race" because people from different gene pools have different levels of vulnerability to certain diseases.
Instead of saying they aren't a thing, I'd rather say that races are blurry, mix easily, naturally change over time, and it's usually insignificant what "race" someone is.
