900 years ago, an alliance of 20 kingdoms destroyed the great ancient kingdom, the most prosperous and technologically advanced. What if, because of technology, this ended in a nuclear war? The 20 kingdoms won the war and settled on the “red line” to get away from contamination, a place 10,000 meters above the surface and therefore very safe.
Joy Boy, however, did not bring the gyojins to the surface because they were contaminated. That’s why he couldn’t do it.
And today the Tenryugu still use an oxygen capsule when they leave the red line for fear of nuclear radiation. That radiation was what caused the different deformations of humans (people with long arms, long legs, long necks, and totally different heights).
Honestly, this makes too much sense to not be the case. There are so many similarities between this story and our actual world.
(theory by Cabreva)