I think it's more likely that LinkedIn mobile app grabs your phone contacts, if you happen to give it permissions to do so.
I've noticed that the "People you may know" section started to contain faceless placeholder entries with emails from my address book (though, I'm not sure if/when I've given the iOS app the address book access).
I've noticed that the "People you may know" section started to contain faceless placeholder entries with emails from my address book (though, I'm not sure if/when I've given the iOS app the address book access).