I believe the hacker discovered his email address simply by clicking on his profile & discovering the email there & the rest is history.
But what does this have to do with their email getting hacked?
While it's wise not to reveal your email when not needed - revealing your Email address isn't a direct threat to its security.
Email addresses should be hidden for privacy rather than security, Email addresses are not meant to stay private or secretly buried in your back yard, they are a just like your public key in bitcoin, your duty is PROTECT the private key/ password.
Every day a few websites get hacked and hackers obtain all sort of email addresses from their databases, the chances that at least 1 hacker knows your Email address are pretty HIGH, so hiding them doesn't do much if they are not secured enough, honestly, with today's security's feature it's freaking hard to gain access to an Email address which is "mildly" secured if you use
1- A strong password
2- Multi-factor authentication
3- Different emails for different purposes
Also DO NOT
1- Use a similar email password to sing-up to any forum/website.
2- Don't use unkown email provider.