PIO isn't limited to FBW aircraft, you can get a 737 into beautiful oscillations as well - all you need is a system with some sort of inherent delay to an input, combined with a degree of dynamic stability - and a pilot doing his utmost to make a benign landing seem dramatic by not anticipating how the airplane will react to his inputs
When NASA was experimenting with the digital fly by wire F-8, they turned up the input delay purposely to study when this would result in unacceptable PIO, made for some slightly scary footage: