It doesn't ensure anything, and I didn't say it did. I did say it typically ensures something, in magic tournaments.
That said, the possible results in a Magic tournament and a Dominion tournament are the same; win/draw/loss. There are fewer draws in Dominion, but other than that I don't understand your point. Low variance? Take 32 players and play one round; you will have 16 winners no matter what the variance. Take 32 players and play two rounds; you will have 8 winners no matter what the variance. etc etc. The variance in individual games/matches is meaningless, you're ending up with a normal distribution no matter what in swiss (draws complicate this, but like I said, they're more rare in Dominion anyway).
It's very easy to predict what the distribution will look like in swiss tournaments, especially when discounting draws and discounting dropouts. With 32 players and 5 rounds, you'll have exactly one 5-0, exactly five 4-1, exactly ten 3-2, etc. In real life it's not quite so easy because of dropouts/draws, but "variance" isn't an issue at all. And like I said, there's fewer draws in Dominion than (for example) real life Magic anyway, so the distribution is closer to normal than what I would be used to from my swiss tournament playing.