Group singing in Africa long predates colonisation. Anthropological work and oral history recordings both point to a settled choral culture already in place, built on vocal homophony and polyphony rather than on a single line with accompaniment. The Ijesha-Yoruba in Nigeria, the Nguu in Tanzania, and the Zulu, Xhosa and Swazi in South Africa all sang this way.
What matters for how the music sounds now is where that singing happened. It belonged to social gatherings, so it was made by everyone present rather than performed at them. A lot of what strikes listeners as distinctive about African choral music follows from that one fact.