I can't speak for PG, but I think the general idea is that a slow influx of new users is less likely to alter the nature of HN as everyone has a chance to acclimatize (avoiding some sort of Eternal September), and the people who really "need" to be on HN (people interested in startups I guess?) will know about HN already, or be told about it. That last part might be a little "fightclub-ish" I guess, but it seems to be working alright.