The point is that sites generally leak this anyway via signup. With github it's even easier, e.g. https://github.com/someuser -- I looked through my settings and their doesn't appear to be any way to "be invisible", though I could be missing something. When you can do a simple query like that, sacrificing UI at sign-in seems totally pointless.

That's the point of the article though - for a large percentage of services, if you know the username/email address you are targeting, you can just try to register them and see if it works. Obfuscating the login process makes little difference.