FWIW if you already have a login on another matrix server (i.e. matrix.org) then as an alternative you can just add the fossi-chat.org matrix server and public rooms to that account instead of having to go through the email registration process (again).

I have a matrix account and am using https://app.element.io/ client. Is it possible to add fossi-chat there? A simple look through the options in the GUI didn't yield a path for me...

Here's where I went in the "Element" client (same place in both Linux and web client as least)