So my company hired this very old guy. The oldest developer I have worked with. I feel they took him just because of diversity.

Because he was absolutely incompetent.

Nothing he wrote ever worked, he got in conflict with everyone, made stupid jokes at meetings, asked the dumbest questions. His stories would hang for weeks, hopping sprint over sprint. Once he delivered something, his code had to be re-written from scratch. A code review couldn't even be applied as the code was worthless, so it just made sense to reassign his tasks to someone else and move on.

So after much drama he was let go. It was maybe two years ago and I recently connected with him on LinkedIn. He's changed four or more jobs since then. Prior to coming to our company he also job-hopped all the time (but most likely he was actually fired every time). His average job duration was like 6 months. Apart from that, he had a 20+ year stay with some government agency.

I fail to grasp how he ever gets hired anywhere with all the red flags. Over two decades with some govt agency (in my country they are all crap). Then change jobs every half an year or so. Then the asshole attitude. And finally, they probably never asked him any technical questions, because he knew nothing. Our interns who were just one month into the job were better by a WIDE margin.