> And there is essentially no discussion of music theory in other cultures (middle eastern, african, east asian, indian)...

To me, that above ^ looks like the biggest gap.

To be fair, the book doesn't claim to demonstrate theory of the "world music" (sic, a phrase as bad as it sounds).

My (brief and shallow) understanding is that most scholars of music in the western world just do not have enough knowledge to talk intelligently about non-western genres. Exceptions exist, of course, but they are quite rare.

[Other reason is that there is a lot of gate-keeping. I've seen that in Hindustani/Carnatic classical studies, FWIW, which is one my areas of interest]