Makes sense I guess, though I honestly find it infinitely more interesting had they all been a bunch of fakes :celebration:
Like I said, we've not yet played Skyrim, so we don't yet know all the details. The source I'm quoting from has a pro-Imperial bias, and may very well be justifying the emperors' right to rule by making them all Dragonborn-via-mantling. Even if they became Dragonborn, I do not see them as being in the same mythological category as someone like the player character or Tiber, who were natural Dragonborn. They are, in other words, the ultimate case of fake-it-'till-you-make-it.
From 2E854 till 3E246 a direct descendant of Tiber sat on the throne and then in 3E246 one a, still blood related though somewhat distant cousin, took the throne and continued until 3E433 when Matrin died.
http://imperial-library.info/content/oblivion-brief-history-empire-1