I would think the Chinese internet would long since have the ability to sniff out encrypted and tunneled packets. Whether the user has the app or not, their data still has to travel over a network rife with snooping.
I would also think that TOR or equivalent networks might be the only thing possible. But of course, those are similarly illegal in China, so whether you get the app elsewhere or inside, it's still going to get you in trouble.
The Chicoms aren't noted for their tolerance of illegal activities.
These guys run a couple of pretty interesting vlog channels on Youtube from China, and they have a good explanation of what’s going on and how it affects them.
https://www.youtube.com/watch?v=EuEdYvQmVFg
China already has a firewall around China. . . which effectively prevents what this intends to do anyway. Google search will not work, for example, nor can a Chinese citizen connect to the US Apple Store. Only specific allowed sites outside of China are permitted. . . all other traffic is either blocked or watched.
As I understand it, only certain specified gateways are permitted. . . and those are monitored. Torrents are not going to be allowed in. Apple already has to store Chinese customer's data on Chinese owned servers. Apple at least avoided exposing their proprietary OSX and iOS source code to the Chinese government, something that Microsoft acceded to long ago, and has done for all later versions of Windows since. Nor have they agreed to permit any backdoors into the encryption on Apple devices.
But the customer data on the Chinese iCloud servers is, apparently, fair game under Chinese law, since the servers are not within Apple's ownership or control, nor do the Chinese people have similar legal protections to those we have in the USA in our Fourth Amendment. In China it is not a good idea to back data up to the iCloud or any cloud. . . but Chinese customers do it anyway.