that book is garbage
I suspect people can't tell the difference between the arbitrage that will happen no matter what (Chicago to New York, etc.) with the front running that came from seeing orders too large for one exchange and buying the rest from the other exchanges so they could sell it to the original buyer.
But Lewis realized that the idea of HFT actually helping the little guys and hurting the big guys wouldn't be as sensationalist and provocative a story, so he decided to twist the facts to suit his own narrative.
Trading is cheap now... because it's all been computerised. The exchanges are digital - as soon as you can do all the transaction processing by computers there is an entire army of (expensive) clerks that can be laid off who used to handle the paper work. HFTs occupy a niche where they can exploit latency between exchanges for profit, with occasional other activities that were considered to be far more questionable when it was people playing those games, not algorithms.
This is true but the (fairly recent) move to essentially free trading for retail investors exists because firms will pay money to brokerages like Robinhood just to get the opportunity to trade with a pool of traders that are unsophisticated.
So why would people pay for order flow? The people who do this are liquidity providers. What they want to do is sit there and buy a stock for $10 and sell it for $10.01 all day long. If they are buying and selling to you or me that is a great deal.
But what happens when BIG HEDGE FUND buys a shit ton of stock for $10. BIG HEDGE FUND probable knows the price is wrong at which the liquidity provider will sell a bunch of stock for 10.01 but then instead of being able to buy it for $10, all of a sudden has to buy it for higher. This is how liquidity providers can lose some of their profits.
So they pay brockerages like Robinhood that don't have big hedge funds as customers, not to fleece those customers but just to sell liquidity to them without worrying about getting picked off.