Keynes was telling a joke. Of course he doesn't think that burying bank notes would be a good idea. Instead he thinks money is better spent on infrastructure, hospitals, and anything that would provide a social benefit at the same time as revitalization the economy. The important thing is to spend money, if the private sector is unwilling to do so then the money must come from somewhere else. He is mocking the neo-classical economists who would have been opposed to government spending, like the new deal. He is saying if you are unwilling to go down the road of the government spending then we will go with this ridiculous idea that fits into the free market economics, and accomplish the same thing the government spending will.