The current debian-cd mirroring instruction page [1] says:

However, it looks like the submission form [2] has been updated recently,
the HTTP path is "hard coded" to `/debian`. There is no `CDImage-*` fields
either. It is not possible to use this web form to submit/update a debian-cd
mirror.

Either the instruction page needs to be updated with the latest procedure to
submit a debian-cd mirror, or the form needs to be changed to allow
debian-cd (or perhaps even debian-ports and debian-archive) mirror submission
or update.