I don't think Gerould dismissed them, rather, he maybe suggested that they didn't/couldn't write them down

This is what you quoted Gerould as saying:

the art of the unlettered portions of European peoples is in another case from that of Bantu tribesmen. They have always formed parts of nations in which artists more or less nourished on conscious aesthetic tradition have at the same time been working.

Which means he knew nothing whatever about the social background of African artforms, which in much of the continent were every bit as locked into monarchical and aristocratic dynastic hierarchies as anything the British gentry and royal court paid for. This was stark staring obvious to many of Gerould's contemporaries, like the European artists inspired by the sculpture of West Africa who knew damn well that they were looking at work dedicated to kings. Gerould had absolutely zero excuse for being that stupid - and perhaps he wasn't: by 1932 African-derived music had become an inescapable part of American culture, and if a scholar didn't want to know where it came from, racist hostility was the only explanation for that indifference.