The title is total linkbait -- once you read through the article, it becomes clear that all the author is claiming is that social login buttons weren't the right choice for Mailchimp. They might be the right choice for others though: "Sometimes it makes a lot of sense, and other times it’s just not worth the trade-offs".

Although Mailchimp has a lot of users, it doesn't make sense to generalize conclusions from one SaaS business to the whole web. A private SaaS dashboard is a different use case from most consumer websites, where the goal of logging in with Twitter or Facebook is generally to attach your public identity to your posts or profile on the site, not merely to expedite login. And frankly, a private dashboard is a very strange place to add social login buttons in the first place. Although the "what login did I use?" issue comes up on consumer websites that make good use of social auth, it does so less often, because if you connected a third-party account to one of these sites, you did it for a reason and are more likely to remember.