This is entirely false, Apple allows the use of threads in their applications.

Oh I found a better explanation

>iOS doesn’t allow apps to fork subprocesses. While on the desktop Tor is running as a separate process, on iOS Tor is hacked to run as a thread inside the app itself. Therefore, you can’t have a system-wide Tor process like desktop and Android. If Tor is running in one app, and you open a different one, it’s not automagically going to start using Tor.

https://www.quora.com/How-effective-is-the-Tor-app-for-iPad-...