Hes basically saying neanderthals didn't go extinct, humans absorbed them as part of our evolution.

Modern humans are extremely insanely diverse. There are billions of us and we are everywhere. We've beaten up natural selection and all had sex with each other.

Neanderthal genes have not evolved for tens of thousands of years. Meanwhile humans have been getting crazy. So we still have those neanderthal genes, and a bunch of new ones too.