The quote explains it. Wealth was not kept as precious metal in a building. It was transferred into bonds, debts, and stocks. People can steal gold. They cannot steal money someone else owes you. I don't know much about the Rotchschilds, but I can make a parallel to Thomas Willing, a colonial financier and first president of America's first central bank. In the colonial days, when people would lend money, their debt was kept on books. If Fred loaned Tom one thousand dollars, or specie, aka precious metal, Fred would have a record on his book that someone owes him 1k. When debt is stored this way, it cannot be transferred or made into liquid currency. People like Thomas Willing and other astute financiers would store the debt as a note. When Tom would borrow that $1000, he'd get a receipt and the receipt could be traded so the lender could be paid back by anybody who had the receipt or the amount of gold it was worth. This circulation of debt receipts created some of the first "modern" currencies. In addition, governments would do business with banks via bonds, which is basically a debt from the government to a private party or institution. I'm far from an expert on financial history, but suspect this shift from gold/silver based wealth storage to debt notes and stock shares is what that wiki quote describes.