Paul Krugman is frustrated because people aren't taking his model of the liquidity trap seriously. OK, let me take it seriously.
Suppose Paul's model is 100% true. And suppose that Paul's model economy is humming along nicely, at full employment, and everyone in the model expects it to continue humming along nicely, at full employment, forever.
Then the central bank does something stupid. At the beginning of period 1, it announces that the period 2 money supply will be cut by (say) 10%, and will stay at that lower level forever. And suppose people believe that announcement.
In period 1 the economy hits a liquidity trap, and there's unemployment.
And Paul would say there is absolutely nothing the central bank can do in period 1 to let the economy escape that liquidity trap, because increasing the period 1 money supply does nothing, so they need to do fiscal policy instead.
And Paul would be wrong. Of course there is something the central bank can do in period 1. It can stop doing the thing that got the economy into the liquidity trap in the first place. It can stop announcing that it will cut the money supply by 10% in period 2, and announce instead that it will not do that.