This guy actually takes his time on doing this stuff, this guy picks his victims through tumblr, this starfish stumbled upon Joel's tumblr and decide to ruin someone or should I say many people's day, once he does he stalks the crap out of the person he's attacking, finds phone numbers, private info like family member names and dates of birth and other private stuff like that he starts his plan of douchebagery.
All he does is just abuses the reset password feature google and many other sites use, you can protect yourself from this by setting up two step authentication and not putting your private information on the internet, even if it isn't that private.
Well yeah, did you expect that he hacked Google to take control of his account? Don't underestimate something so simple. It probably takes him months of investigation before he can find enough information to get someone's account. The reason it happened to Joel is because he wasn't careful enough. If Pewdiepie was as careless it would've happened to him by now, being basically the number one target on YouTube for this kind of stuff.