>Isn't that a very solved problem?

Not in my experience. Most solutions I've found tended to be more work/brittle than just emailing the file to myself, which is a sad statement about ad hoc file sharing.

Other solutions like $ python -m SimpleHTTPServer or spinning up an ftp server work okay too but sometimes you run into NAT problems if you're traversing e.g. residential home networks.

Magic wormhole is simply the best way to get files between arbitrary points A and B, irrespective of operating systems or NATs and firewalls in between. I've never had it not work.

I don't think it's limited to a local network. I believe you can copy files anywhere. See https://github.com/magic-wormhole/magic-wormhole

The GUI people then need to fix their first sentence "Wormhole-gui is a cross-platform graphical interface for magic-wormhole that lets you easily share files, folders and text between computers on the local network."

Or am I confused again?

Good point. I might have to update that. I have never actually tried transferring between two networks and did not want to advertise i in case it didn't work.

It does indeed work outside of the local network. I have updated the documentation.