I dunno. If the Universe is infinite, than an infinite number of possibilities exist.
Nirn exists. The TES universe exists. Even in "our" universe.
All of you are proof of the above two sentences. It's just not the existence or reality most think it is. The TES universe has a clearly defined state of existence and reality under which the TES universe operates. Multiple sources inside the lore refer to that fact and some even try to describe it and teach it.
As to the original question: The TES universe, or Morrowind, can't be put into a single category of earth's history. Just because there are swords in it doesn't make it medieval. As has been mentioned alreaedy, elements have been taken from Ancient history, over the Middle Ages to renaissance and back. Technological aspects have a vast range and cannot be compared to earth's technological development stages. Spaceships have been known even while people still use swords, for example.
At the very least, only small aspects of the game can be compared to a part of Earth's history, but in TES's completeness, there is no single period of earth's history that it can be compared to.