The Chinese firms were nationalized by the government in July, giving full access. This isn't generic surveillance but complete ownership. First source listed in my original comment.

Dragonfly was complying with China's firewall, because clicking broken links in search results sucks, but it was also linking every search query to the users phone number and sharing with the Chinese government, which is what Google employees revolted over.

But since key management is out of your control or visibility, Apple can just add another key to your account on behalf of the government. They don't have to disclose the existing keys on your device. This gets them the messages going forward but not the ones from the past. So they'd have to do this for all Chinese customers ahead of time, rather than as a response to an inquiry.

Your messages are stored on iCloud, right?