Earlier quoted context omitted.
Money is the thing you use to buy things. Wealth is having those things. Only in an ideal world are the two the same. The world is not ideal.
But why couldn't they, e.g. buy other people's thing in exchange for their gold? IOW, why is the relevant fact that _Spain_ didn't have much more wealth despite having more gold? Other people in other countries valued (and still value) gold too.
Also, to quote Jastram [0], "In spite of the romanticism of the Spanish Main and treasure ships, the supply of gold from the New World was just a trickle by later standards. Less than 1% of what was to be 1930 world production was produced in each of the years from 1500 to 1520." Over the next 80 years, it never got much above 1.3%.
[0] Roy W. Jastram, The Golden Constant (1977), pg. 41.