What do you mean by local users - users on the same LAN or different user profiles on the same phone? The latter is not supported. Whether it works at all or not isn’t entirely clear, I remember both users reporting it does as well as it doesn’t. And even if it works, I’d be very surprised if android let Syncthing instances of different users run at the same time, so that they could communicate.
And I am probably misunderstanding anyway, because I don’t see how airplane mode comes into all of this.