Ultimately It all comes down to power, and perception of that power. For the sake of argument, lets say there was a group of people utterly ignorant of magic. Anyone with the ability to throw fireballs or otherwise kill on a whim, turn invisible, heal people from the brink of death or even resurrect them (or their corpses at any rate) could be pretty hard to separate from the aedra or deadra, especially considering how relatively little they actually interact with the world. At what point does a man ascend to godhood? The moment the majority believes he does. Likewise what would the gods be if no one believed in them, and if they could exercise little to no power in the world? Nothing. So in the eyes of the thalmor, Talos is not a god. In the eyes of nords, he is most definitely a god.