I'm not in agreement with Sally on that one, either. I
have fermented successfully with a tight fiting lid, but I often get more mold that way, too. I usually follow Katz's recommendation of a loose covering, and with less mold to show for it, too.
It just doesn't make sense to me to ferment something without exposing it to the air, which is where it would get the organisims it needs to ferment properly.