The day you arrive is "Day 0," not "Day 1."
So if in fact you leave on day 91, you have not overstayed.
I doubted this statement from the moment I read it. And a few days ago I had the chance to take a peek at a passport for someone who had recently traveled to the US on the VWP. The entry on 11/21/12 expired 2/18/13. The entry on 3/21/13 expired 6/18/13 (expiration is printed in the passport). For both of these, the end date is 89 days after the start, leaving 90 days total including both start and end date, not 91. Unless you have any sources for your claims, this indicates that you are giving incorrect information/advocating ilIegal activities (overstaying), and I must ask you to stop that.