Depends on how you define atheism.
Corypheus existed, claimed godhood, had worshippers, supernatural powers, immortality, etc. If, hypothetically, all of Thedas acknowledged him as a god, would you consider it be reasonable to not view him as a god?
Mythal and Fen'Harel appear to have existed. Lets say, hypothetically, they have all the powers attributed to them and their legends are real. Would it be reasonable to not view them as gods?
Say the Maker exists, and created the setting and everything in it. Would it be reasonable to not view him as a god?
Say Zeus existed here on Earth, would it be reasonable to not view him as a god?
Personally, I think whatever people view as the requirements for godhood(which generally tends to just be abnormally high power or special abilities) in a setting to qualify them is the most reasonable path. I don't think atheism makes any sense in say, Forgotten Realms. If you refuse to call them gods at that point, you're simply arbitrarily using a different definition than the rest of society for no purpose other than qualifying yourself as an atheist(which is super meta all things considered).
It also bears mentioning that acknowledging something is a god doesn't mean you have to worship it(in many multi-deity settings people only worship one or a few of the gods, not all), any more than acknowledging something is a cat means you have to pet it.