Outside of a few wealthy democratic enclaves, the world is largely a place where the rule of law means the rule of the strongest, where human rights don't exist, and where everything and everyone can be bought for the right price. This includes international waters.
Because thinking of human evolution leads to all sorts of horrifying thoughts and those kind of timescales can be ignored given the rapidly approaching spectre of AGI or even just human genetic engineering.
It's ignored because it's horrifically inhumane and also probably not even true. If you earnestly have this thought, I urge you to seek help from a psychiatrist.
I don't think he was saying it is good that "unfit" children die in mines, that would be a pretty horrific assertion.
I think he is just wondering why we don't consider the impact of immigration on genetic diversity. If we have nations that have huge immigrant populations and nations that have huge numbers migrating away from them it would have an interesting impact on genetic diversity. There are a range of traits that influence whether someone would migrate from a place or stay where they were born. I don't think pondering the effects of that requires a psychiatrist.