I think what he meant was that your email address is the most likely to be used as an identifier with, for example subscription websites. It does have the advantage that you can confirm it.
But what about your identity? I have several email addresses and handles online, only one has my real name in it. From the rest you could deduce my nationality, my interests, a quirky sense of humour and possibly my city of residence.
More accurate than an astrological chart or a Cosmo Quiz.