That seems a mistaken assumption. The presence of existing comments often causes me to refrain from posting comments. Often I've been about to reply to something, then notice someone else has already said substantially the same thing, and as a result don't.

And this is not a contrived example. As I said when I first launched pending comments, if someone says something important in a nasty way, it may well turn out to be safe not to endorse it, because someone else will probably show up and make the same point without nastiness.

Plus if HN gets more civil, it may encourage people to comment who might not have before. I know from my own experience that incivility has decreased the number of comments I make. I often find myself about to say something, then decide I have work to do and don't have time to spend the afternoon fighting, and so don't say it.

Only experience can tell us whether one or both of these factors will increase the number of comments, but we can't assume that pending comments can only reduce discussion.