The whole premise of the David and Goliath story Lewis wants to tell is that HFT is a device used by the Wall Street establishment to screw over the rest of the market.
But in fact it's more the opposite: HFT firms are upstart prop trading and market making shops that are pissing off giant banks like Goldman and JPMC by making it harder for them to skim profits off block trades for pension and hedge funds.
Also: the use of rhetoric like "dribbling retard" isn't what makes an argument "ad hominem". An ad hominem argument is one whose logic rests on a claim about its opponent and doesn't account for its opponent's argument. The logic behind the quote you're commenting on doesn't do that.