Are you suggesting that Africans learned to harmonize from missionaries, and were not singing in harmony before contact with such missionaries?

I don't think that's at all what Marje is suggesting. More along the lines of cross-fertilisation when two styles of harmonies met. As the words and tunes of certain shanties were formed from the fusion of African and European elements, so also with the harmonies.

I think that when Marje referred to "the missionary tradition that took English church music out to the colonies", the colonies she had in mind might have been Virginia, Carolina, etc.

For a couple of hundred years before the shanty explosion, slaves would have been exposed to the religious music of their white masters, so by 1820 would a purely African style remain intact, totally uninfluenced by contact with Europeans? Or would it have become Afro-American style(s)?