let the hacking begin!
You did not read the part about these devices join on other networks, not the Internet. The way it works is the network only has a few or a few hundred computers on it an no connection at all to the Internet. So you not be able to send emails to most people from you classed iPhone, you'd use your other personal iPhone for that.
Ok some tmes these networks are connected to the Internet by some kind of link. Mostly the link is an "air gap" where you'd have to put the data on the thumb drive, get it approved and thenove the drive. The rules are usualy complex and vary between organizations
So, no one in China is going to hack your secure iPhone for the same reason that can't hack the CPU chip in your car, because there is no comunications path