I agree - two things I will add -
1) I was recently reading an essay which very convincingly cites numerous slave-era historical sources with very clear references to African retensions - and as the essayist poinst out, slaves were still being imported to the US in the late 18 and early 19th centiries, so Africa was not such a distant memory -
2) Never underestimate the power of an oral culture to disseminate cultural ideas even at a great geographic or chronological distance - I was listening, a few years ago, to a CD which contained a recording by Eddie Anthony, an old black fiddler who recorded in the late 1920s - suddenly, out of nowhere, I heard a bizarre phrase, a way he had of shooting his finger aloing the strings to produce a piercing high note that was, in relation to the harmony of the piece, a complete abstraction - and what floored me was that I had heard, on more than one occasion, the trombonist Dickey Wells play the EXACT same figure in NYC in the late 1970s -
very interesting -