>Apple kept conformance with SMS/MMS standards from the beginning and added a secure layer on top.
iMessage is not a "secure layer on top", it's a totally separate proprietary protocol and it requires an Apple account to work. It just happens to run in the same app as SMS/MMS messages, which has its pros and cons.
“The “magic” is that you don’t have to sign up for an account, or create a new username or account identifier. You just send a message from your phone number to another phone number, and if both numbers are registered for iMessage, the message goes over iMessage instead of SMS, even if you don’t have an Apple ID. Beeper had that working last week. Now, Beeper users need to have an Apple ID, and sign into that Apple ID within Beeper. (Beeper should actively encourage users to create and use an app-specific Apple ID password[1] for Beeper.)”[2]
iMessage is not a "secure layer on top", it's a totally separate proprietary protocol and it requires an Apple account to work. It just happens to run in the same app as SMS/MMS messages, which has its pros and cons.