Cash and negative interest rates
Actually they're saying that, when cash is available, people need not to deposit their money into the bank, and hence is "guaranteed" (not taking into account risks associated with holding on to cash, like fire or getting stolen) a 0% interest rate.

Having cash, therefore, means that if the deposit rate goes into the negative territory (you're charged to deposit your money), then people could simply stock cash at home. As a result, the interest rate could not go much into the negative territory.