Earlier quoted context omitted.
For that to be true, you'd need to establish that blood oxygen levels differ significantly across altitude adapted individuals, and I don't believe that this is the case. Also, when I first moved from Philadelphia to a village in New Mexico at 6000', I remember reading that I should plan on a 3 phase adapative process that would take as much as 9 months to complete. This makes me give less weight to things like the 2…
Why would you need to show that? I don't see anything that requires blood oxygen levels to be significantly different to get from "people at high elevation have access to less oxygen" to "living in lower-oxygen environments creates the right conditions to see serotonin go down and dopamine go up". Even if oxygen levels are identical once you get to the blood stream, it can cause many changes further up the process.
Your cells cannot measure environmental O2 levels - only what is available from the bloodstream. Ergo, if sufficient adaptation has taken place - and it may not have - then as far as the body is concerned, there is no depleted O2 environment.