I assure you these numbers are real. I have no inside knowledge, but I have submitted a few StackOverflow posts (with my username attached to them) that made it to the front page, and I watched how many of them were from my link on HN.
"Stuck due to “knowing too much”" (http://news.ycombinator.com/item?id=4037794), 68 votes, about 15,000 visitors
"What's The Best Language For Safety Critical Software?" (http://news.ycombinator.com/item?id=3943556), 108 votes, about 25,000 visitors
(there was another one with about 200 votes, but I didn't find it!)
Compare these to submissions like this one, that has 350+ upvotes already, and I think it's realistic to assume at least 60 or 70 thousand unique users (I mean users in the sense that they know HN and visit it consciously) visit HN every day.