Note: The exceptions to this rule are certain kinship nouns that come from Arabic: baba, mama, bibi, babu, shangazi, binamu, rafiki, dada, kaka, etc. These nouns take the y- agreement in the singular and the z- agreement in the plural.
Where do you get the idea that words like dada and kaka come from Arabic. Likewise mama, baba, and bibi might be of questionable origin (whether borrowed or Indigenous). Only rafiki and binamu among these examples.
It's basically animate nouns in the N class. Otherwise you'd use wao for both plural and singular and since the nouns of this class don't show number, there'd be more cases where number cannot be determined.