The boards are only a fraction of the total player base. I think the only people on my friends list who also post on the board are the people I added because of their board presence. And I can't recall ever recognizing a name on the boards from my games.
Then, most posts are complaints. And that's not unexpected because it's much more common for someone to speak up over things they don't like than things they don't. And most people complaining about bans wait until after they've gotten 2-week and permabans. I don't think I've seen more than 1 or 2 after a chat restriction. And only a handful of the posts actually ask for help/advice on improving thier behavior.
The rest just seem to want to complain, make excuses, and try to justify why they didn't 'deserve' the high-level ban. They want permission to treat people like crap, and that doesn't fly here. Unfortunately, they are also some of the loudest voices on the behavioral system.
I've been around the behavioral sections of the boards and the forums for a long time. I've seen hundreds, if not thousands of posts claiming they "always" run into feeders, trolls, and afk/leavers. I may not be playing as much as I used to, but that's more a case of time and internet quality than disgust over the player base (and yes, that's a major reason I rarely play more than placements in ranked). And in the games I do play? I rarely see people I think are worth reporting.
So, yes. I am inclined to believe the given stats.