They can’t shut down distribution of source code, but:

- your typical user wouldn’t know what to do with it

- every install would run only for a limited time (only apps distributed through the App Store have a certificate that doesn’t expire), so those users would have to reinstall the app every week or so.

- it would make it harder for them to make money from it (they would give away the game for free. The in-game store wouldn’t give away stuff, but the source likely would soon be changed to support alternative stores)