I'm not quite sure about that. As much as I trust Nu-Hatta on things like mantling, incarnation, and Towers, there's too much evidence to suggest that Humanity originated in Atmora. If all life originated in Tamriel, it would falsify so much history such as Ysgramor's Return. It would mean that humans would have come into existence in Tamriel, sailed up to Atmora, and then sailed back into Skyrim. It just doesn't seem logical to travel so far when you could just stop and build civilizations at the nearest river or coastline.
But to comment on the OP, while the Redguards have a distinct belief system that isn't quite anologous to those of other races of Man, they are human.
This is exactly what the Anuad tells us though.
They left, and later returned.
But those that left were not men, but Ehlnofey.
"A large fragment of the Ehlnofey world landed on Nirn relatively intact, and the Ehlnofey living there were the ancestors of the Mer. These Ehlnofey fortified their borders from the chaos outside, hid their pocket of calm, and attempted to live on as before. Other Ehlnofey arrived on Nirn scattered amid the confused jumble of the shattered worlds, wandering and finding each other over the years. Eventually, the wandering Ehlnofey found the hidden land of Old Ehlnofey, and were amazed and joyful to find their kin living amid the splendor of ages past."