Ok ok, jitsi works. But its not that nice to use for CG. Here are the steps:
File -> Enter Chatroom, Enter the chatroom name followed by @conference.codingame.com
Select „More“ and input your nick. Teilnehmen.
Once entered the chatrooms stay in the main window.
Bonus: Private Chats.
They are a bit complicated as you have no buddy list, and the user list in the chat rooms does not contain the correct targets. You can get the id from the browser developer tools. Easiest way is to start a private chat on the site, then see in the xmpp-websocket for the message. They contain the id.
With that select File -> Add Contact, enter the ID and a Nick.
You can not get the presence until they have allowed you to add them, which is not possible from the website.