The reason you aren't allowed to publish applications containing GPL'd source code in the Apple Store isn't because the GPL forbids it, it's because Apple forbi

The reason you aren't allowed to publish applications containing GPL'd source code in the Apple Store isn't because the GPL forbids it, it's because Apple forbids it; they have an unnatural hatred for the GPL. From what I can see the Signal application uses GPLv3'd code. I have no idea how it is still available in the Apple store.