No, I think we get each other. There's two different issues here, and what I'm talking about there isn't the one you're talking about. There's some reasons you might not play to win in a casual game, because you're playing just for fun. I mean, in a casual game, Sally has won 5 in a row, I'm more apt to try to make her not win the next one, even at my own expense, just because I want a change in the winner's circle. Jimmy helps Cindy because she's his girlfriend. I take the last province even though I had a 1.3% chance of winning, just because I want to get it over with and move on to the next one. In each of these cases, the competitive environment is going to make me want to play to win more, which means not so much to bump off Sally's win streak, not to help a significant other (well, probably you don't have them playing in the same game), and not just wanting to end it, because there's still SOME chance - my time is comparatively less valuable here.
Or, on the other hand - the competitive environment affects how you play, but it or may not affect how other people play. People will still go after Sally, Jimmy will still help Cindy, someone will still take the last province and end the game with a loss.
In casual play, it's not a problem, it doesn't matter. People played cards, there's a eventually a winner, everyone had fun.
Whereas in competitive play, it matters, because suddenly the winner MATTERS, the winner mattering is what makes it competitive in the first place. Maybe there's money on the line, maybe just a fancy title or pride, or whatever. And in that case, Jimmy helping Cindy at his own expense is anticompetitive and a problem, if Cindy ends up winning the tournament or taking the pot or whatever, with Jimmy's help, at the expense of all the other people who may have been better Dominion players than Cindy but didn't have a helpful boyfriend passing them Colonies at critical moments (or helping in more subtle ways).
Of course, the competitive play does add in the possible meta-gaming issues if you give any kind of credit for non-wins, and you don't have that in a one-off game, but only in some format where multiple games are being tracked.
People can always get credit for non-wins, no matter how you set up the rules to prevent it, because you can set up a friend with a win and then share the winner's prize after-the-fact, whatever that prize is. The only way around that is to make nobody get any sort of credit for wins OR losses - i.e. just have casual play.
