I fixed the xmpp authentication system yesterday, it was getting blocked by an anti-spam system from my xmpp provider (and they don’t want me to reveal details about what I had to change, so that real spammers won’t know).
I think for xmpp clients that don’t support this xep, you can simply reply to the message with the confirmation code, but I have not tested this. If it works I should add it to the sent message.
I tried first in Monal and wasn’t sure what to do so I did try replying with the code itself but maybe it wasn’t the first thing I wrote. It didn’t work. I’d have to try with another client and make sure the first reply I send is the code.
Checking the code for the authorizer, indeed I didn’t write code to handle that case, actually.
I can add it if using clients that support showing a dialog (such as Renga itself) is a problem.