That's wrong information:
On this page[1] nic.at states (under "Cancel a domain") that cancellations can be made by fax or letter.
Also on the "fax confirmation" PDF[2] it says in big letters: "Please send it by fax or scanned by e-mail to: ..."
[1] http://www.nic.at/en/service/
[2] http://www.nic.at/fileadmin/www.nic.at/documents/formulare/C...