Both iMessage and Whatsapp are using XMPP which does in-fact support store and forward of IM messages - hence it's being used by these services.
The only additonal part is the background notifications which use Apple's (or Google's on Android) proprietary system. But once you start your client app, they connect to the server via XMPP and use that for message delivery.
The only additonal part is the background notifications which use Apple's (or Google's on Android) proprietary system. But once you start your client app, they connect to the server via XMPP and use that for message delivery.