Please stop with the hyperbole, weak defaults are not backdoors, know the difference. A backdoor is precisely what the UK was commanding, that is a secret way to siphon the cleartext data while purporting E2EE. Messages and other data syncing to iCloud using standard encryption (that is both you and Apple know the key) is simply a different trust model. It has its own set of benefits because there is less technical burden on the user which would usually arise in the event of a disaster and attempting to recover the data.

Yes, because Apple knows the key, they can yield your cleartext data to law enforcement or whoever they authorise, but that is implied in the trust model, it isn't kept a secret or done dishonestly. A backdoor would is a devious or dishonest mechanism to siphon data, which nothing of the sort is happening here, because the trust model is transparent to the user.