iMessage is E2E even without ADP, even with groups and multiple devices. The details are complex, but they are publicly documented here[1]:
The issue (I think) you are referring to is that if you enable iCloud backup[2] or iCloud for Messages[3] (both of which move effectively move the storage of the messages to the cloud, either as part of the device backup or as the canonical representation that devices sync from respectively) then the messages decoded on device will be stored in blobs that iCloud has the keys to unless you enable Advanced Data Protection.