That's my understanding as well. My original reply was in response to your suggestion that iMessage differentiate RCS encrypted vs RCS unencrypted using colors, and it appears that wouldn't even be possible because Google is applying proprietary encryption on top of RCS using their app.
I find this absolutely hilarious and almost beautiful: Google has been harassing Apple to implement RCS because it's an open standard and because its users feel green bubbles on iMessage are exclusionary. Now Google has implemented a proprietary protocol on top of RCS that only works with its messaging app, and only messages sent between users of that app appear in a darker blue color with a special lock icon.
So Apple will ship the RCS standard in iMessage, and communication between Android users and iMessage will be sent using RCS, but iMessage users will appear to Android users in the lesser light blue bubbles alongside the dark blue bubbles only given to Android users with Google's proprietary app! Huzzah!