Tiber Septim was long dead in Morrowind. Morrowind took place in year 427 of the Third Era, which was founded when TIber Septim united Tamriel. I think you mean Uriel Septim.
=.= I've already edited my post about that, tired minds make clumsy mistakes.
A new thought to bring up is what someone said before.
Daedra are immortal, so the ones you found in Morrowind were likely there for a very long time, until you killed them off.
Were there ever other cases of Daedra actively spawning in Morrowind?
Also, about how Familiars, Atronach and Dremora can roam, is because they can be summoned by a mage or prince, but they cannot freely walk through a portal without assistance. And, I don't even think Familiars are Daedra.
So wouldn't it be possible too that there would be a lot of roaming daedra from old times in Skyrim ? Also mages should be able to summon all the other kinds of Daedra too. Not so sure about if the Familiar is a Daedra myself really, this is the first I see of it in TES to be honest and it feels kinda random or even out of place for me. But for older fans who know the lore better they may be making a welcomed return to the series, I just don't really know.
The whole point I am getting at however is that the situation in Skyrim is not so different from the situation in Morrowind when it comes to the seal protecting Tamriel from Daedra.
And Golden Saints weren't creatures.
They pretty much acted as such in Morrowind. (Or are we speaking like non-daedric, what I mean here is that they were not NPC's like in Oblivion but acted like any Daedroth or Hunger in Morrowind)