Being fired: Happens quickly, they either give you a reason or tell you "your position has been eliminated." Sucks, but rips the Band Aid off and done is done.
PIP: I've seen this happen to good and bad employees. Bad employees - it just drags out the process and provides a lot of friction and anguish, for the manager and maybe the employee. (Depends on whether they are actually even aware they're a bad fit or not, and whether they're well-intentioned or not.)
Good employees? It's literally gaslighting people. One day they're pulled aside and told they're not performing well enough and given a PIP. The PIP usually isn't designed to help them get performance up - it's creating a paper trail to manage them out.
Suddenly (as described in the parent article) they're being hit with complaints they've never heard before. Maybe there's substance, maybe not. It's a major blow to self-esteem and it's a dragged out process. They're not even given the satisfaction of a quick firing where they can be angry and done. It's a long suffering.
And to make the managers do this stuff - it's awful. Having to lay somebody off when a company RIFs people sucks. Having to fire people usually sucks unless they've really earned it. But telling a manager they have to PIP a percentage of their team arbitrarily and participate in this gaslighting process is simply evil.