I'm not sure I understand the point being made. Chomsky seems to be saying that since we learn the prevalent morality of our culture through sparse data there must be an underlying universal morality. But how does that allow for the fact that different cultures do have different moral codes and that those do change over time?
As he adjusts to his new situation, his behavior changes. The moral code of rural Oaxaca is different them that of east LA. His moral code changes. Perhaps he always gave a great amount of help when needed to neighbors in rural Oaxaca, but stops doing that as much. This changes. In Oaxaca, he did not steal from his friends, nor does he in LA. This does not change.
It's obvious even in US culture. Two students go to Harvard, one goes into engineering, one diplomacy. What group are the moral traits of candor, frankness and directness valued? What group are the moral values of politeness and courteousness valued? Morality can be relative, depending on the situation. But for some situations it is not, the more universal things.