IMHO, the argument is that Apple does not allow any third party messaging app to send message to the built-in messaging application (Apple Messages) on the iPhone. That is Apple ships one messaging app that is the default and may not be removed, and they also do not allow any interoperability with that one messaging application.
Apple Messages is not an SMS application; it's an internet messaging application that falls back to SMS messages when communicating with any non-iOS device. There are some situations where there may be no data network and, maybe, it falls back to sending an SMS message to another iOS device but this is pretty rare.
Apple Messages is not an SMS application; it's an internet messaging application that falls back to SMS messages when communicating with any non-iOS device. There are some situations where there may be no data network and, maybe, it falls back to sending an SMS message to another iOS device but this is pretty rare.