So someone else besides me finally thought this up? Awesome!
My theory is slightly different, though: Keep in mind that some more primitive versions of modern "Chimans" existed before the coming of the Phoenix. The Phoenix Tribe was originally just another avian tribe from one of the other lands on Chima beyond the "floating island." The Great Beings uplifted them to what they are now, but before they could do that with the other tribes, they heard about the Core War and left the Phoenix to complete their work on Chima.
Chi may be modified Energized Protodermis: Creatures that are exposed to EP don't have any control over what they become, while those augmented by Chi gain humanoid bodies. We can go back further and say that EP was an early nanotech experiment by the Great Beings that they basically left in the cores of some planets and forgot about. Chima was one of them. When they came back to it, they discovered the EP and further modified it into Chi.
I apologize if this doesn't make any sense; when I read the quoted post, I remembered an old theory that had, got a brain rush, and tried to fill in some of the theory's holes before posting it here.