When trying to create a secure network, you're constantly going to be dealing with tradeoffs between enabling communication, and ensuring you can actually talk with people.

You're right that iOS isn't a completely secure OS.. But using a secure app on iOS is better than using regular SMS going through AT&T.

Perfect is very difficult to achieve here- Most PCs have nonfree a BIOS, and even then, many CPUs can be updated by encrypted updates from the manufacturer.

It very well might be possible to ensure that your machine isn't vulnerable... But you're not going to have many people to talk to.

I think the tradeoff for having an iOS app is worth it. It puts the users of the iOS app (and those talking to them) a bit more at risk, but doesn't compromise the whole network.

Let them make that tradeoff. It's better than talking to an empty room.