Because you need to open some port on your router, create an SSH account, and let your friend connect to your machine. That's a pain point.
From the video it looks like this works via SSH as well so you will still need to "create an SSH account, and let your friend connect to your machine"
You don't need to create an SSH account or let your friends connect to your machine. Your session is proxied through the tmate.io server (safely) so that you can avoid the headache of opening up a connection.
This is tmux with the headache of sharing an ssh session solved.
so share ssh token instead of share ssh credentials?