I understand him perfectly, what I'm saying is what he's proposing is useless, because you can mold your email and password into anything that you know for certain it will pass validation, that's your oracle right there. During sign up, if you know both the formatting rules of what you typed as the email and the password cannot have caused an error, then it follows any error message (500 errors notwithstanding) must be a duplicate email. Because that's typically the only constraint difference exists between an email and a password.
Given that, who cares what error message you display, as an attacker I only need to be able to find out weather a system has an email or not, and the sign up screen is a perfect oracle.