To add RSS feeds, you need to be logged in and you are redirected to this URL. And if you go to the signup page, you can login through Github. After you do that and try to add the RSS feed, you are again redirected to login page. So, it's not creating an account like the normal signup process does.

Take note of the starting URL.

Here is a GIF of the entire experience:

More Info:
Fedora 27
Tested on Firefox developer edition and Chrome