I don't think the subreddit is less representative.
It has a vastly larger population, and they skew a lot younger.
Also, the way you're portraying is not accurate. It's just what's going on there right now, it hasn't always been that way. Reddit's voting system means issues blow up like solar flares then gradually quiet back down. For example, a few years back, the dominant "extreme opinion" on the subreddit was "Rolling stats suck and you should be ashamed to roll stats" (some people made some kinda-good arguments that way too even if I was arguing against that extreme of an opinion).
And if you're saying you've "never ever seen a more fervent community about the caster/martial disparity" then either you're being forgetful, or you weren't on ENWorld back in late 3.5E, because bloody hell mate, what's happening on the subreddit is absolutely nothing compared to that. There was a point when LFQW was probably one of the top phrases used on ENWorld, when it was in practically everything.
So yeah, they're not less representative, they're more representative. But they're still not necessarily very representative.