I am very fond of this interpretation. The opening graphics kind of imply the Haixingren were there, but all the intro says is that "some" of the people in the spaceship mutated to take on the characteristics of plants and animals, and some went underground and gained powers. It's Daqing who says half those in the spaceship were Yashou, and the other Dixing, but by the time he's speaking about it, the spaceship landing is already at "age of legends" distance.

Plus, Shen Wei and Ye Zun grow up topside, with the strong implication in Shen Wei's backstory that they were born there, so it seems the Dixingren were originally in both places. And then when Shen Wei wakes up, he pretty much immediately returns to the surface. So, when he says Dixingren "don't need sunlight," how would he know? I mean, okay, professor of bio-engineering, and yeah, the implication when the Lantern is restored is that it serves as the source of "sunlight" for Dixing, so they would have had that until the Hallows scattered, but still. His assertion just doesn't seem to have any basis, and his own reaction to coming back to the surface after the pillar would argue that Dixingren do need sunlight. Maybe not as much as Haixingren, but it still looks like a mood booster, at the very least.