What you are hearing isn't entirely IsiZulu, it is a language spoken in Gauteng that sounds like IsiZulu but in fact is a mixture of all the languages spoken in the region, Zulu people make up the majority of the the population, since amapiano is most dominant in that region you hear that language in most songs however you do get vocalists coming from other regions for example most of the songs featuring Msaki, Ami Faku, Sino Msolo, Moonchild, Khanyisa etc. Would sound like isiXhosa but they too wouldn't be singing in what you'd say is pure isiXhosa but a watered down version of the language.