The confusion is that there is only one texting app on iPhone. Chat apps are done "over the top" and can be whatever you want. You or I can make one. There is only one texting app on iOS and most users in the US only use their phone's texting app. This is why Apple's iMessage is genius, insidious, and diabolical- because they took SMS which had universal adoption in the US and had it invisibly and transparently extended into a component of their walled garden. They didn't need to convince everyone to move from SMS to their own messaging app, because if you used SMS on an iPhone, iMessage just happened.