Yes. Thats true. There's something called inbreeding depression where the smaller your genepool gets the higher the chances of disabilities get. But that's usually in relation to a small group of people.

We for example see that in Europe. Its widely known here that Arabic people marry inner family. Not brother and sister but usually cousins. If you repeat that for generations you get issues. I have to see if i can still find the article but Germanys muslim population is 5% but almost 30% of all kids born with a disability are from Islamic background (I'd have to look up the exact numbers again but it was a lot lot higher than it theoretically should be (5%)).

However - when we're talking about millions of people its generally not an issue. Besides that - what Americans classify as white is actually quite diverse. We have the nordics, the germanics, italiacs, irish, greeks etc etc. They all differ genetically.

So while there is not any genetical harm in mixing races its also not like the races need to be mixed to stop something like an inbreeding depression when we're talking about a group with millions or hundreds of millions of people.