From a pure endurance sports point of view, natural ability of latin americans in altitude has been successfully reached by other athletes through altitude training camps, tents simulating altitude and drugs (epo,...).
I mean it can certainly help, but this is still well within an average human's range of adaptability. Building up new muscle "easily" (and also atrophying muscle when it isn't used) is one of human kind's super powers in the animal kingdom.
You aren't going to run into any real significant physical limits from your genes until you are pushing beyond what the top 1% of other humans can do, and being able to run up and down mountains all day isn't something only a portion of the locals could hope to achieve, native to the area or not, they just gotta do it for long enough.