If it were a single order in the book, I'm not sure I see the advantage HFTs would have conceptually had. They're not sniffing individual orders and then outrunning them before the original order hits the exchange; they're exploiting the individual piecemeal orders that are stealthily trying to move the block.

They're hunting down the orders of informed traders who are trying to exploit their own knowledge of the true price of products, and correcting the price on the market before those informed traders can take their knowledge out of everyone else's hide.