It doesn't matter that much whether or not Sifferkoll used Thomas' registration e-mail to look for info (though it would make things worse).
It also doesn't matter whether or not Thomas' info is easily accessible on Google.
It doesn't even matter that Thomas was using his real name (sounds like victim blaming)
Thomas didn't want his personal information to be posted here. There is a small chance that doing so carries negative consequences to his private life.
I see this as a form a bullying/harassment and the owners of this forum are complicit.
Legally this is also a grey area.