Signal and Syncthing are great. magic-wormhole is more about setting up that initial connection: when two humans know each other, but their computers haven't met yet (i.e. know each others pubkeys).
In Signal, the security of the initial message exchange depends upon the phone network (did somebody spoof caller-id to claim Alice's phone number?) and the Signal servers (did they report the correct key for Alice's phone number)? Once you've verified keys in person, those concerns go away. In magic-wormhole, the security of the initial message exchange depends just upon the wormhole code.
For Syncthing, if I remember right, you have to exchange "Device IDs", which are like public keys. You can send them over email, but the security depends upon the email servers (did any of the servers along the path replace that DeviceID with a false one?). The window of opportunity for that attacker is basically the same as it would be for a wormhole code sent via email. And Syncthing gets you long-term/repeated sharing of a folder, whereas magic-wormhole is one-shot.
My goal for magic-wormhole is to offer it as a provisioning protocol for other tools (with better UI and more functionality). Imagine if Syncthing had an "Invite A New Device" button, and pushing it gave you a wormhole code, and the other Syncthing instance had an "Accept Invitation" button where you type in that code. Then you could get all the nice UI and workflow of Syncthing, but you wouldn't have to transcribe the large Device ID (pubkey), and you could do it over something safer than email (like a phone call or just speaking the code to the coworker sitting next to you).