I agree that’s not how it works in most places but I don’t assume to know the inner working of a Chinese iphone or the version of iOS it’s running. If there is a financial incentive apple will bend for China while also saying it didn’t.
The way Facetime works is extensively documented and thoroughly audited by third parties - many of whom publish their findings.
If China had a back door into Facetime, I suspect I’d know about it as someone who follows these things pretty closely.