Steve: Perhaps because Gibb is primarily concerned with singing, work SONGS.
Playing an instrument might well have the same or similar effect but it is something different.

An instrumental is not work SONG because... no lyrics. Say what?

Gibb presents a very persuasive case that chantying evolved directly from African-American activities.

Without ever once using the word "celeusma" in a sentence, ergo... "Needs improvement." Considered and eliminated or was the author utterly and completely ignorant of the subject matter?

I've read your post history too Steve, same questions.

I'm not arguing what you or Gibb are "concerned with," only the validity of using "likes" as a metric for naval science.