A God is a deity that creates and/or controls the cosmos. A deadra does neither.
Sure you can call them gods, but being a god is not a title of authority and power, and by nature they are deadra.
Maybe they are, as mentioned before, gods over their own realm.
Then only the Godhead is the only "god" in your definition. And even then, I'm a little wary on that, because it didn't really even create anything, only went into a schizophrenic episode and is still a completely shattered mind.
Also, the definition of god in TES seems different from the definition you seem to be using. Plus, the princes didn't create their realms, they are their realms, literally. The aedra can't be gods, because Mundus and Nirn are just parts of them, cut up and mashed together. The only creation they did was cut themselves up enough to create a spherical body for the lesser spirits to reside in, with the gift of free will.
Lastly, there's nothing in TES that went "'and let there be light' and there was light." or something along those lines.