These numbers are based off the current price of hashrate. As soon as you try to buy significant amounts for larger currencies like Bitcoin, the numbers skyrocket.

The nice-hashable column on the site shows how much hash power is available for purchase.

To read the page naively would be a little like claiming one could hoard 51% of the oil supply given today's price at the pump. In reality, as soon as you started buying in large quantities, the price would skyrocket (making that attack very much more costly), suppliers would cut you off, and others would notice.