Someone who is a jerk will be fired very quickly when showing incompetence. The bar is much higher for a jerk, and even then he can be supercompetent but still get fired because he is a jerk and not good for the team. Someone who is nice gets much more leeway, but if very incompetent will still get fired.

So the problem with the reasoning in the article is discussing niceness and competence as dichotomies, while in fact they are scales and you can be much more incompetent when you are nice, but there still is a limit.