...after his supervisor [] ranked him, in [], as the worst performing person on his team, despite him bagging the biggest bonus that quarter, and earlier meeting or exceeding performance expectations
That was my experience. (Not IBM) a company that says it's a meritocracy and all that hogwash. The problem for me was that I was in the way of my colleague's promotion prospects - easily fixed by manager chum.
That prompted me to move team - a previous team leader was more than willing to get me back, I knew my days were numbered as all newcomers were millennials - and there were only a handful of developers of my age range. When I was let go, the cohort that left were the older ones. A couple of months after, the very experienced team lead left as well, as he could now see the writing on the wall