>Humans have not really had enough time as independent populations to diverge much
There is no scientific basis for that claim. Many other animals have diverged less, in less time and are still separate subspecies.
>African > Eurasia had regular exchanges as they where within relatively short walking distances.
Interbreeding does not mean subspecies don't exist. Coyotes and wolves are totally separate species and they interbreed. This does not make them the same species, much less the same subspecies. Or the very example given in the article: polar bears and brown bears. Separate species, overlapping geographical regions, crossbreeding occurs in the overlap. Or again from the article example: giraffes.
Also note they talk about the genetic distance, but the genetic distance between a west african and an australian aboriginal is greater than the difference between wolves and dogs, which are subspecies. Again, by all measures we use for wild animals, humans consist of several subspecies at least.