What Poropat, Duckworth, and others suggest is that multiple traits - including "grit" - contribute to success. He even provides evidence to back up that hardly-surprising conclusion. So how does Kohn respond? By immediately projecting a "one trait uber alles" mentality onto the grit proponents. To be even more clear, he's attributing to them exactly the idea they're trying to refute. Then he cherry-picks examples of excessive persistence leads to adverse outcomes, ignoring the issue of whether those outcomes would be likely to occur in people who had developed other traits such as curiosity and openness. In the end he only demonstrates further the problems with any single-trait theory of learning, supporting exactly the point he meant to oppose.
Maybe his parents or teachers should have helped Kohn develop some more of those other traits. Like honesty.