No, we gave him plenty of interesting work. And as we discovered, the above described interview was actually a fairly low bar; when faced with problems slightly more challenging, he would fall apart and give up in frustration. And he was very touchy, so wouldn't accept help when he hit those kinds of situations; he would just spend all of his time reading Reddit and HN instead of getting the help he needed. He was essentially impossible to work with.

We had overlooked his personality quirks when hiring him, as we didn't want filter out someone who was technically sharp but was simply personally quite weird, but it turns out that in practice we just could not work with him.

That's possible, but it's equally possible from the information given that she (*he as it turns out) was just lazy and unmotivated or was in some other way actually the problem. Both types of people exist in great numbers.