syncthing and Magic Wormhole have different goals. Wormhole is very simple: it is a way (often the best way) to get a file from point A to point B, regardless of the connectivity (or lack thereof) between A and B, without accounts or configuration.
Syncthing also does relaying like that, NAT traversal, bridges, etc. all automatically. If it can, it will use local IP connections and go blazing fast. If for some reason your network configuration(s) don't allow for any valid connections, it will use a relay or bridge - either a default or one you setup - and work relatively fast as well.
"Connectivity" here needs to be taken at large: syncthing needs to have a folder already shared between two machines. There's some setup involved that is not automatic.
The entire setup phase of magic wormhole is "copy those 3 words" and boom you're done.
I think a lot of people talking about Synchthing must have used it in a different way in the past. It has a QR code now that encodes that 16-digit number or whatever that is used for the same purposes of key exchange and initial handshaking.
I use it with my wifes phone to transfer files between her drawing tablet and her Linux system she uses for Blender every day.
Yes, but even that is more involvement: you need to create a new specific folder, add a device, share that folder, accept on the receiver, copy the file in the folder, and wait for full sync in the web ui. It's a much longer setup that makes sense if you will synchronize files/folders with the same machine in the same folder, but that's not always the case. magic-wormhole removes all that back-and-forth dance for one-off sends.
