A lot of people have seen the influence on Howard’s thinking of the Hyborian, Mu, and the lost Atlantis continents from Dr Wegener’s ‘Pangea’ theory, which his 1915 book ‘The Origin of Continents and Oceans’ first has the supercontinent idea, referenced as Pangea just the one time. So many postulate that the Hyborian age is a few hundred million years after the initial break up, hence Mu being so far out to the west (possibly Australia pulling away), the inland sea separating what would be Europe, Asia, and North Africa. And the Pictish wilderness as the proto-Picts of Howard’s later historical time frame stories with king Bran Mak Morn, after a splinter group of survivors headed to the Hyborian continent from those ‘far western islands.’
Howard himself was mute on the specific subject as far as I have seen. That said, the native American style of weaponry, war making, and token’s seen in stories like ‘Beyond the Black River’, where North American tribe names are used by Picts, is also a bit of an indicator that the Pict’s that migrated to the west in the Hyborian age were possibly also a race that mixed with others to start human civilizations there, thus influencing what would become Native American culture. Also Howard’s physical description of some Picts in Conan’s time can be interpreted to have what we might consider features of the native Americans north and south.