Games exist on many other platforms than these. My partner and I used to play quite a lot of TM online before it was implemented here. In addition, plenty of people could just be playing the physical game - it's quite reasonable to have a weekly game night where you play your favourite game at least once every week, which can add up to a lot of non-online play.
The idea of the higher K factor initially is to more quickly get people to the rank they belong at (hence wins give you big swings up and losses big swings down). But for people who've played very minimally on BGA, you can't really assume anything about their ability.
Additionally, I've seen players who can just grasp how to play a game very quickly and dominate other (casual) players. They're not likely to beat a seasoned expert, but most players aren't expert in more than a few games (because of the amount of time required to get that good at any one game). I don't consider myself expert in much at all, but I have a few friends who when I show them new games, more often than not, they'll still beat me, even if I feel like I have a reasonable grasp on the game, and sometimes I'll spot them employing strategies that hadn't yet occurred to me despite having played a significantly higher number of games.