That's really not true. Almost all of his caustic responses were to people sending him shit. His post on the Steam forums was in response to trolls trashing his game. Just take a look at his forum posts.[0] Most are completely normal, while one or two are replying to jerks.

Fish isn't exactly a saint, and I'd venture to say he's quite a jerk (I'm not a fan of the shit he said about Japanese games.[1]) But the number of times Fish has dished it out is vastly outweighed by the number of times he's taken it.

[0]: http://steamcommunity.com/app/224760/discussions/search/?aut...

[1]: http://www.eurogamer.net/articles/2012-03-07-fez-creator-phi...