As others pointed out, I believe the 3.4% was simply down to social logins introduced much later. When I fist signed-up for mailchimp ages ago, the only option was creating a new user account.

I think the article dismisses one huge benefit to federated logins:

* ease of use for users - instead of choosing a username, entering all the customer information, verifying the email address etc, choosing a password, you can sign in with one or two clicks.