I think what they meant is "Without inflation there is no incentive to invest your money, rather than stuffing it in your mattress"
If the currency is stagnant or close to stagnant, the incentive to invest is that you end up with more wealth overall.
I'm not sure it's possible for deflation to be both predictable and higher than the market rate of return, because people today would immediately speculate and bid up the price of the currency to reflect its future expected value.
Of course inflation is an added incentive to invest money, or at least to store your wealth in assets other than money.
It's just paper. What's wrong with it sitting in someone's safe? We certainly don't want actual productive capital assets to be idle, but dollar bills don't produce anything. If they're sitting in a safe, the value of the ones that aren't in the safe goes up slightly.