Encrypted with shared keys. This wasn’t accurately described in the comment you’re replying to: iMessage is fully E2E but the offload feature which allows you to save space on your phone wasn’t. That helps in the lost phone scenario but not if you’re worried about subpoenas under a hostile government.

Prior to the recent advanced protection program, you could avoid that by not enabling backup for messages. Now you can choose whether data recovery is more important than E2E to you.