Earlier quoted context omitted.
the reason for it is specifically addressed in the article... "But after some further consideration, we decided that it was a false risk, as the username reminder form already tells you if a username exists"
For usernames it might be true but many sites use emails instead of usernames (and rightfully so, it's already complicated for people to remember passwords without forcing them to also remember an unique username). Emails are more personal and might be easier to link back to personal information. Thus, confirming that there is an associated account with a given email is also a privacy leak, because maybe people don't…
And as they say in the article, that information is already leaked by the password reset process: "No email by that name available".