I think (but this is just a guess) app authors can distribute otherwise-GPL apps on the App Store by distributing the App Store version under a different license (even if it's exactly the same code). I don't know whether or not contributors would have to agree to a different contributor agreement, though...

I'm basing this guess on Qt's licensing. They distribute a GPL version of their software, but allow users to buy their software if they want to use it under a different license. I'm assuming Joplin could do something similar for app store distribution (but really don't know).

Edit: Oops! Someone already posted something similar to this up above! Sorry for re-posting!!!