Tibet really isn't on the map (maybe the edge), and it did not have a large population at the time.

Plus, EB doesn't add units just because they would be cool or such. Find some historical evidence that the Tibetans left Tibet and went west looking for jobs as military mercenaries, then find out how they fought. Do this and the EB team will make a Tibetan mercenary if they trump all other units that deserve faction slots.