If one group's settings is FollowingOnly and they click on the Follow This Topic link at the footer of an emailed message from the group, as expected they are taken to the Follow this Yes/No page (which btw still uses "thread" instead of "topic").

If one clicks on No there however, it does the same thing as Yes does, adds it to your FollowedTopics list and lands you in the FollowedTopics page.