True, but if you read a bit of Scott's writing it will become apparent that in this respect he is better than average, by a long way. Of course being good at writing is not the same as being good at talking, but I am willing to trust his opinion that he's good at talking too. (He's a psychiatrist by profession and presumably does a lot of Talking To People About Stuff.)
Fun Ask HN thread would be, "at what thing or what about you is below average?" It's one thing to admit you suck at something, but quite another to recognize that you are below the mean, or worse, the median.
It's too easy to fall into the trap of inverting a positive trait and say, "I'm probably less un-awesome than average," or to exaggerate false humility.
Sample bias comes in as well, since among my rich friends, I am below average net worth, but still above the 75th percentile nationally. It's like that stupid job interview question, "what is your greatest weakness?[0]"
It's an interesting exercise anyway.
([0]Apparently one wrong answer is, "suffering fools gladly.")
Scott Alexander (SlateStarCodex) is most definitely above average when it comes to writing and argument-making. He might be the most convincing writer I know of.
Everyone thinks they are better than average ;)