I think you hit the nail on the head. Liberal boards are pretty much as he describes them. Either flame fests or boring drivel. His mistake is that he assumes that because liberal political boards are like this, that all political boards are like that.
It would have been pretty easy for him to skulk around FR for something nasty to post. So why didn't he take the easy shot?
I could assume he has principles (knowing that most discourse here is reasonable and not wanting to give a slanted view). But he's a liberal, so I'm going to assume that he didn't want to give us free publicity.