There is limited information, and even the TOS crew comment that these visitors might be the basis for those myths and stories, not completely accurate to what these aliens would be like, with some exaggeration perhaps coming in to the stories.I'd have to watch "Who Mourns for Adonais" again, but other than what "Apollo" claimed, do we even have evidence the other Greek gods even really existed? I'd question how much of Apollo's claims to being the mythological being are even valid at all.
I guess there's Olly as evidence others of Apollo's people existing, but their actual role in Earth's history is pretty vague and I don't think you can assume the canonicity of say, The Iliad.
Kirk and company do not seem too fussed by Apollo's claims. They're willing to accept it on it's face, and note much of the cultural touchstones from Greek history as being a basis for future development. While I'm sure their interaction in human history was probably limited, I don't see a reason to doubt Apollo's claims, even if they're not 100% accurate.



