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.

Which is why they shouldn't leak this information on the registration page... Instead they should pretend the registration always succeeds and then disallow login until the email has been verified.

That works for the email, but not the user name. Going through the entire process and then telling the new user "That one's already taken, please start over" is not going to make people happy.