I don't dispute the underperfomers are the typical case. But I've seen plenty of glaring exceptions. The point is: things aren't always what they seem. And we can't automatically assume that the people who make these decisions are making the right decisions, or even know what's happening under their noses.
BTW if they put the guy on PIP then that basically means they were in the process of firing him already. So if they called it a "layoff" that was for cosmetic purposes.