Were ancient gods myths, or could they have been visitors from elsewhere?

The myth-and-culture explanation needs fewer assumptions. Deities often reflect the concerns, values, and natural forces that mattered to the people who worshipped them. The alien idea is interesting speculation, but a story describing a powerful being isn’t evidence that the being came from another world.

I’d separate two questions: what a deity meant to the people telling the story, and whether some extraordinary visitor might have inspired it. The first can be explored through the culture and its texts. The second would need independent evidence, and a godlike description by itself can’t establish that.

People can enjoy the ancient-visitor theory as a thought experiment, but it’s easy to read modern technology into old stories. If the claim is that these were aliens, I’d want evidence beyond dramatic imagery or unfamiliar descriptions. Without that, mythology is the more grounded way to discuss the stories.