i'm a fan of E2EE, but the AT&T hack did not involve intercepting RCS messages. It's sort of like saying "Chewbacca is a Wookie, therefore you shouldn't use RCS."
I don't see the section that says it involved accessing RCS messages.
See my other comment. Gruber doesn't say it involved accessing RCS, but he does write nearly 2,000 words strongly implying and reinforcing that "Apple knows best" and that if they say they shouldn't have to support RCS, you should trust them.