Interesting idea.
So far, I assumed that the giant-kings built their kingdom first, and accepted the serpentpeople later as refuges, just as they accepted the Lemurians long after.
But let’s investigate this.
The serpentpeople are a very ancient race. We do not know if the giant-kings are an even older race; both is possible.
The giant-kings are followers of Set, just as the serpentpeople (see below). Would make sense if they’re related, and be consistent with the serpentpeople being their progenitor race.
The serpentpeople did try to breed with non-serpentpeople. The game only talks about couplings with reptilians, but who knows, may be possible.
The serpentpeople live right next to the frost-giants. The serpentpeople probably wouldn’t like the icefields the frost giants inhabit and vice versa, but hey, people accept a lot of hardship for some snu-snu, especially with a sexy frost giant or an unusually, uhm, flexible serpentperson.
The Staff of the Triumvirate says something about their relationship:
" Though they would never admit it when I dwelt among them, they had the Serpent Ring of Set, stolen from my peo… the serpent people who went extinct thousands of years ago."
That would also fit with the serpentpeople being older, the original followers of Set and maybe the cultural and biological ancestors.
The description of the Shining Trapezohedron, howerver, goes against that:
"Perhaps fleeing their genocide at the hands of the legendary Kull of Atlantis, they made their way to the Exiled Lands where they befriended the Giant-kings through worship of their deity, Set. In return they were allowed to live in the Caldera of the volcano and build their cities there. "
This assumes that the giant-kings already lived in this land before the serpentpeople fled there. It’s a speculation, but…
… the Isle of Siptah strongly supports that. On of the vault lorestones says:
“But then Others of our kind made contact from the mainland. They had made a pact for safety and secrecy–hidden beneath a volcano in the lands of the Giant-kings. They had birthed a crèche of young.”
It is still possible that the giant-kings somehow are descendants of the serpentpeople, but at least they claimed the Exiled Lands before the serpentpeople came. Else, they wouldn’t refer to the lands as being the giant-kings’.
So… I’d not support that theory, at least not without further evidence.