As has been clear throughout Game of Thrones' three seasons, the core of the Lannisters' power is their immense wealth, which supposedly greatly exceeds that of any of the other seven kingdoms. Slate, however, is arguing this wealth, though immense, is practically not as valuable as what House Tyrell has to offer. They cite a speech in which Warren Buffett made the point that all the world's gold could be melted into one 68 cubic foot cube worth $9.6 trillion (now it would be worth $7 trillion), which is nice, but for the same amount of money, one could own all the U.S. cropland and sixteen Exxon Mobils, with a cool trillion leftover. Adding: "In Westeros, the Lannisters have the cube of gold and the Tyrells with the rich farmland of the Reach have the real resources." To put it simply: "You can’t eat gold."
Now you can imagine Tyrion arguing that gold never spoils and the Lannisters have enough to buy wheat (pssh, wheat) for three eternities. Sadly, Tyrion, it's not that easy. In a scenario in which the Tyrell's gold disappears, they could survive on their crops until some sort of exchange for riches can be arranged. If the situation were reversed and the Lannisters found themselves sans food and trying to "trade a whole big pile of gold for a whole big pile of food," then "the price of food will skyrocket." The idea of the Lannister's wealth is built on the belief that we can just "multiply [the marginal price of an ounce of gold] by the total quantity of the Lannister gold supply, and then conclude that the Lannisters are hyper-wealthy." Where "in reality, any effort to mobilize all that metallic wealth will lead to inflation rather than the ability to mobilize vast quantities of real resources."
They conclude by saying, "Real resources — not shiny gold — are the true test of wealth and the real source of power." Fair, but no: Dragons are the real source of power.