Though it can fall back to SMS in case you don't have data, which isn't E2EE. I'm not sure what the UX flow is like in that case, whether it warns you and asks for permission to send over less secure channel.
I was under the impression that this is a 'proprietary' extension between Google devices, and that there was no RCS-standard-based E2EE:
> The RCS specification defines several types of messages. Our implementation of E2EE uses varying strategies for encrypting each type of message to maximize user privacy while still adhering to the RCS specification.
Though it can fall back to SMS in case you don't have data, which isn't E2EE. I'm not sure what the UX flow is like in that case, whether it warns you and asks for permission to send over less secure channel.