That was intentional; the wording in https://www.mediawiki.org/wiki/Requests_for_comment/Process suggested that email conversation should be pursued orthogonally to discussion on the Phab ticket. The phab task links to the email discussion archives, so nothing's going to be lost.
If you're in the habit of using phab for discussion, you didn't need to see this email: phab would have notified you as soon as I created the task in the RFC project. The mailing list thread is for people who would prefer to discuss (or ask questions) outside phab.
FWIW, [slightly offtopic] I find discussions only on phab tend to have an echo chamber affect, where only the author, and people who the author regularly collaborate with see the ticket (Generally speaking). So I'm glad that these sort of things have mailing list posts too.
-- -bawolff