Hence why I said “not built in” — also per the document, “all RCS chat messages will be E2EE if both clients have the latest version of Messages” which means if an attacker with enough resources blocked the target end-point and inserted a device without the add-on E2EE encryption, sounds like the messages would be sent in the clear, which to me is not end-to-end encryption; that is unless both parties get an alert the messages will be sent in the clear, preferably before they are, using prior method a nation state or telecommunications company at their order could likely easily read all the messages and forward them on without either party knowing. Hence, SMS & RCS need to die.

Am I missing something?

PS- Assuming that document is by Google, but unclear, what is the source that links to it?