Not that amazing. Considering that Vinge is of high interest to a number of HN readers, that there are a fair number of HN users, and that Deepness in the Sky is of sufficient length and density that a readthrough (especially a first) can take a pretty decent length of time, I wouldn't find it at all surprising is there was always at least one HN commenter who either was currently in middle of or had just finished Deepness in the Sky.