A good PhD leads to many of the nice things he describes: freedom and ownership and personal growth and all that stuff. An average PhD (or worse) leads to pretty much the opposite. Most of the superstars I know went on to do pretty much whatever they are interested in at top schools. The non-superstars (and the real lumps, like me) can easily wind up in a death spiral - where your mediocre publishing record and mediocre PhD afford entry only into 3rd-tier institutions, where you will work with worse and worse people, more or less guaranteeing steadily declining quality of work. A mediocre result more or less guarantees that you will be a low-status drone in academia, trying to wedge world-class work in with a bunch of other activities (teaching, being a glorified research assistant, and other 'service').
No-one sets out to do a bad PhD, but people need to understand that the average outcome isn't nearly as glowing as Karpathy outlines. Similarly, the outcome of going to industry also has a huge range. I found myself immediately - I mean on Day 1 - doing more pure Computer Science going to work for a startup than I had any reasonable hope of doing as a semi-failed academic, and have had a steadily improving experience subsequently (some of this stems from a rather delayed growing-up on my part, so it's not entirely a judgement on industry vs academia).