Nobody uses iMessage in China because SMS is expensive.
iCloud data in China is stored in a datacenter in China where the Chinese government can access any data any time they want with no warrant whatsoever.
As for the second sentence, iMessage isn't iCloud. And even if the iMessage data is stored in the same data center (which I have no idea), it's end-to-end encrypted so that doesn't help China.
iMessage isn't iCloud, but iMessages are stored in iCloud, Apple also stores iCloud encryption keys in China. See https://techcrunch.com/2018/02/25/apple-moves-icloud-encrypt...
To buttress your belief, is there any circumstance where they didn't cave to requests from the Chinese government? Or anywhere where they publicly disagreed at least? Honest question as I'm unsure if it has ever happened.
> To buttress your belief, is there any circumstance where they didn't cave to requests from the Chinese government?
I don't see how I could have an answer to that question, because Apple doesn't publicize the times that China asks them to do something and they say no. I would certainly imagine that China has probably asked them to break the encryption on iMessage.