It was a huge mistake.
Turns out when all other options are exhausted, a PIP can be a form of respect. In the hands of the right manager, an underperformer gets one last clear chance to show they can do the job. If they succeed, some the ugly baggage gets put behind them. And if they can't succeed, then the PIP sends a message that they are unsuited for the job at the company, and maybe even at the industry at large.
The alternative to PIP'ing an incompetent (not just an underperformer) is micromanagement. That comes with pretense, hostility, and disrespect to their person for an indefinite period of time.