I think it's just a default Google thing. Basically, it notices you're logged in but you haven't entered your password for a while, so it prompts you to login again.

Instead, I just clicked 'Log out and log in as a different user', closed the tab, and clicked the link again on HN, and it worked just fine.

So in short, it doesn't require you to be logged in, unless you are, at which point you have to log in again.

Bizarre.

I wonder if most people didn't even notice that because they remain logged in to Google services all the time.