Spock did describe as living among "other humans" that implies they are an Earthborn race. Pelia has also only ever referenced her time on Earth as far back as Pythagaros and not once brought up a homeworld. They have never been referred to as visiting Earth, refugees on Earth just living there. While absence of evidence is not evidence of absence the suggestion that they are from Earth has more going for it than them being from elsewhere.
That's my impression as well. That they're just a splinter of humanity with very long lifespans. Not extraterrestrials.
Not unlike Flint in "Requiem for Methuselah."
He wasn't an alien either.
