1. The DNA is consistent with the Kilwa Chronicle, a text probably recorded from an earlier oral tradition that was written off as being not based on history
2. There's a strong sexual bias in the admixture. Often that happens as an unpleasant side effect of colonization by patriarchal societies (eg Spaniards go to Latin America, impregnate a lot of local ladies, and as a result the Spanish DNA tends to be passed down from the male side and the indigenous DNA from the female side). There's a similar effect here, but it might be caused by traders trying to ingratiate themselves with the local matriarchy.