In fairness I think he literally wrote the code the way he claims he wrote it 'as a rant', so it's not surprising he didn't actually test it. If this was written off the cuff without testing it's actually pretty impressive.
I know he would have caught these bugs had he needed to implement these for a real system. But there is nevertheless an important point here. He is 100% right that over-architecting is a bad idea, and that correctness is more important than architecture. But complaining about a factorial function not handling negative numbers, and then "fixing" the problem by implementing the Gamma function, is doing the same thing all over again. It's really easy to fix the factorial implementation to do something reasonable for negative numbers; it's much harder to implement a Gamma function that works for all input. Why bother unless you really have to?