They might be able to relay them in a way that the end to end encryption is actually handled on the phone and the relay only relays encrypted messages.

They'd need to control the app on both phones in order to control what it's encrypting/decrypting. Their system only works because they've got a device in the middle separately decrypting/re-encrypting each message. Google's Messages app can't read iMessages; Apple's Messages app can't read Google's proprietary encrypted RCS messages.

Of course if you want universally cross-platform messaging, complete with full-resolution photos and available with end-to-end encryption, there's this crazy new technology called "email." I feel like there's a missed opportunity for making setting up S/MIME easier.