The author suggests stripping the part before doing the uniqueness check. This does not mean that the username (email address in this case) would not be allowed.
Obviously it never works, as I get the "I see you're trying to create a new account" email, but one of these days he's going to figure out a way to take over one of those accounts and then I'll really be fked.
(1.) What are you imagining is the attack vector exactly?
(2.) Are you asserting that all website owners should build to Google’s (non-standard) behavior?
Can me and my wife both sign up to HN and use my email but hers be josh+swife@joshmanders.com and mine be josh@joshmanders.com?
That's a strange usecase, isn't it?