>> My guess is that you think iMessage is SMS-only
> No, there's Apple's proprietary protocol...
Earlier you asked: "But iMessage is already open?"
Now you are saying that iMessage uses "Apple's proprietary protocol". I hope now you understand that when people say that Apple iMessage is not open, they are not talking about the SMS protocol that Apple does not own.
You can send an SMS.
>My guess is that you think iMessage is SMS-only
No, there's Apple's proprietary protocol, that you can only use on Apple devices. But from non Apple devices you can use the standard SMS.