He could do this because he saw the IP addresses of forum users, and this poster was using their office computer.
(If you know someone's IP address, you can go to a website like this one and possibly get some idea who they are. For domestic users, you'll probably just learn their ISP, such as Virgin, BT etc. If they are at work, and work for a large organisation, you might learn their employer).
Many forum posters protested that this was unfair, an abuse of mod powers etc. I agree, but the mod in question made a good point in reply - that you are always representing your employer. It may not be nice, but it's a good answer because it's true.
I was reminded of this when Birmingham MP Sion Simon made headlines today by making a joke about swine flu on Twitter. He doesn't hide his identity, so it's not an exact parallel. But the lesson is that he can never be just Sion, talking to his mates, on Twitter or any other part of the Internet.
He's always going to be MP Sion Simon (Lab Erdington). Which probably has a moral for us all, hidden there somewhere.
This is what he said (actually re-tweeting someone else)
I'm not saying Susan Boyle caused swine flu. I'm just saying that nobody had swine flu, she sang on TV, people got swine flu.
I did speak to one Tory MP to see if they wanted to be outraged, but they burst out laughing and insisted they didn't want to comment . . .