iMessage and FaceTime are available in China, but it hardly matters because most communication in China happens over WeChat. If all the important stuff is happening over the heavily monitored app, leaving a few scragglers is not a big deal. Especially because, it's not like a criminal gang is going to have the money to arm every member with an iPhone with China's wages (and the ones that do are likely white-collar and detectable elsewhere), and it's also not like Apple is allowed to actively advertise or even discuss themselves as being "more secure than WeChat."
EDIT: Also, ~80%-90% of iPhone users in the country likely have iCloud Backup on. iCloud Backup is not E2E-encrypted in China, and is hosted by a Chinese company instead of Apple. If you want full E2E, you need to use iMessage and hope everyone in the party doesn't have iCloud Backup on... and that's a pretty niche threat now.