That is so absurd. I think so many people lie about their PRs on the web, a hugely warped perspective of average ability has developed.
Looking at race results of 5Ks in the Chicago area, a 15:30 would win 99% of those. That time would be good enough for All-Conference at your average Div. III track meet. I'm not sure when they switched from 3 mile to 5000 in Div III, probably around 1978-80. Since that time, my old college has only had about a dozen runners go under 15:30. With an average of 4-5 runners doing the 5000 every year, that's a pretty small percentage.
At best, the average times that average MALES between 18-40 could achieve, in ideal circumstances, around 19:00.
Keep in mind that the average Americans are not athletes. If you randomly selected a group of ten 18-40 year old males, chances are none of them would be runners. Even if they had a great coach and 3 years of training, chances are slim that any of them would reach 15:30. If a couple had some talent, then maybe they'd reach 16:00-16:30, with the rest ranging from 17-21 minutes.