You seem to have one of the more common misconceptions about end to end encrypted messengers. The end to end encryption is used for messages in transit from Alice to Bob, once they've reached Bob's device they're generally stored unencrypted (albeit usually with encryption at the system level). Encryption of stored messages and backups is a separate feature.
The only reference to iMessages stored in iCloud I could find in this document is the following:
iCloud also backs up information—including device settings, app data,
photos, and videos in the Camera Roll, and conversations in the Messages
app—daily over Wi-Fi. iCloud secures the content by encrypting it when sent
over the Internet, storing it in an encrypted format, and using secure tokens
Here’s what iCloud backs up:
- iMessage, text (SMS), and MMS messages (requires the SIM card that was in use during backup)
So it appears that all data backed up to iCloud is encrypted, but the key is almost certainly held by Apple. Since the SIM card is required to restore iMessages that hopefully means that iOS encrypts iMessages with a key derived from information on the SIM card before sending the data to iCloud, but I haven't seen any confirmation of this, and I'm not familiar enough with SIM cards to know how secure this would be.
As for syncing messages without a backup, all I see is:
The user’s outgoing message is individually encrypted for each of the receiver’s devices.
Which isn't so much "syncing" messages as automatically sending the same message to all of the recipient's devices. If the recipient activated a new device after the message was sent, it wouldn't appear on that device. This doesn't seem to line up with your claim of being able to sync and recover messages without an iCloud backup.