He means the 20th century. Governments thought they could solve the debt problem with austerity but people simply responded to the lower interest rates by literally keeping their money under the mattress.

This is still true today. Interest is at zero, people keep their money in bank accounts of banks who are so heavily overleveraged vs their demand deposits that they are unable to take the risk of lending.

Money is stuck and has to be replenished by more borrowing i.e. QE asset purchases give banks more reserve assets and let them pretend their balance sheet is clean.