Polls aren't certainly the best possible reflection of what a community thinks if there is no perfectly representative a sample of participants. But it can be a good aproximation. Electing representatives is a kind of poll, so is HN itself. Participants vote here and there, factions balance out, you get some unstable equilibria that does the job of reflection part of what a community thinks and it often helps a community take better decisions than what a few (interested and absolutely politically factioned) politicians usually do.