The Mathemagician says of course the answer is way off. The point of the page is to demonstrate that you can't make long-term conclusions based on current trends. He says he wants to show people that you can't reasonably assume that things will always be (or have always been) "as they are today".
Of course population hasn't always been growing at the current rate of around 3% per year. Only a hundred years ago or so, far fewer people lived long enough to get married and have kids, so the rate of increase was definitely less.
Recently, the Mathemagian says, he's seen an example of this kind of false assumption when people assumed that since house prices were rising, they would keep rising and it made sense to buy more house than they could afford. We've all seen just how well that worked out.
I think I get it now. The Mathemagician's point is that when you are trying to predict the future, based on current trends, you have to be careful about considering whether the current trend is really a fundamental indicator of how things work, or just a temporary change from the norm. He always tells me to look at these sorts of extrapolations and see if the results look silly (like they do in the Adam and Eve case). If the results look silly, then the new trend may only be a short-term thing. I should look more closely before basing big decisions on it.