"Is that true? From my experience Africa is perhaps the most likely place where traditional music is going to be naturally harmonised with the minimum interference from western influences: http://www.youtube.com/watch?v=y3g15n9qCdc"

The musical example you cited on You Tube is more modern rather than traditional. The drumming might reflect the traditional earlier styles but the harmonies are influenced by missionaries and hymns.

If you listen to recordings of earlier African music, you will hear more monody or unison style singing with appropriate effects. Afro-pop, church music and other European influences have found their way into African music such as "High Life" and other forms influenced by American jazz. Traditional music from Dahomey or the Ituri Forest show little influence if any of Euro-harmonic tradition.

The way in which some kind of harmony enters the picture is when you hear a kind of polyphony in the voices such as with the Pygmies in the Ituri Forest but this is in no way
a Europeanized harmonic form as we know harmony from pop, jazz or classical music.