The argument being made here is about Katsuyama's complaint that the prices he'd see in his terminal would suddenly move away from him if he tried to trade large blocks of stock at that price.
What makes Katsuyama sound incompetent is his apparent lack of understanding of the price pressure his own large block trades put on the market. Lewis makes Katsuyama out to sound as if he can reasonably expect that if FCOJ is quoted 29.78-29.80, he can sell 100,000 shares at the quoted price. Of course, not only can he not expect to do that, but the influence that large trade has on the market is the whole reason Katsuyama is paid by RBC to trade for its clients.
Hence: either Lewis doesn't understand his subject well enough to report on it, or Katsuyama doesn't understand the basic function of his job.
In any case: not an ad-hominem, sorry. There's a popular Internet fallacy that says any argument that uses insulting language must be an ad-hominem. Insults aren't usually productive, but they don't automatically produce flawed arguments.