iCloud Backups are enabled by default, and the Messages in iCloud key is in the non-e2ee iCloud Backup.
That means the iMessages being synced are encrypted to an endpoint key which is held by both the endpoint and the middle transit service (iCloud/iCloud Backup). That's end-to-middle-and-end encrypted, i.e. not end to end encrypted.
Even if you turn on iCloud Backup e2ee (it's an option now) then your iMessages to everyone who hasn't (99.9%+ of people) aren't e2ee because the other end of the conversation is backing up their endpoint sync key.
That means the iMessages being synced are encrypted to an endpoint key which is held by both the endpoint and the middle transit service (iCloud/iCloud Backup). That's end-to-middle-and-end encrypted, i.e. not end to end encrypted.
Even if you turn on iCloud Backup e2ee (it's an option now) then your iMessages to everyone who hasn't (99.9%+ of people) aren't e2ee because the other end of the conversation is backing up their endpoint sync key.