But, if that's what were happening, there should be a noticeable effect before the leader comes in.

This result, if I understand what they're doing correctly, implies that things are going about as well as anywhere else until the populist comes into power.

Considering that protectionism and soaking the rich are mainstays of populism, that's roughly what I'd expect to see in an experimental setting, but it would be weird if there were no economic trends preceding populist movements.