Sideloading also allows authoritarian regimes to add apps to your phone. See https://www.theguardian.com/world/2019/jul/02/chinese-border...
https://www.macrumors.com/2021/03/16/apple-to-offer-governme...
Something Apple still doesn't let you do.
Certainly, once a device enters into China, it should be considered breached and insecure.