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.