The one thing that completely rules this possibility out is the matter of how physics and astronomy work in the Mundus. Our stars aren't views into Aetherius, nor is space Oblivion, nor have I counted two moons up in the sky any time recently. The climates are similar because Bethesda Softworks is run by human beings like you and I and decided to draw inspiration from, well, Earth. Would you prefer if Tamriel was in tricolor like a Dr. Seuss book?
EDIT: Interestingly enough, this reminds me of that fantastically innovative LucasArts game
LOOM. Despite its presence of magic, divination, and a rather medieval setting, a tombstone suggests that the game takes place in the 8000s. Possibly our own Earth following a destruction of humanity and its eventual rebuilding, now once again in the middle ages. That whole "mankind over-expands and soon collapses, and the cycle of history repeats again" thing.
Then there's also all the magic and semi-physical gods that are ever-so-slightly conspicuously absent from our world. So basically, no, it is not in the least bit possible for Nirn to be our world in any way, shape, or time period.
What a fool you are.
I am a God!