The buyers make money by either taking the spread on an order, reducing risk in their own portfolio or using the order to create impact and internalizing the tail.
The author goes on to say "Robinhood is well on their way to making hundreds of millions of dollars in cash income by selling their customers' orders to the HFT meat grinder. High-frequency traders are not charities. The only reason high-frequency traders would pay Robinhood tens to hundreds of millions of dollars is that they can exploit the retail customers for far more than they pay Robinhood."
You can probably back out what the HFTs margin is on this stuff by looking at public financials... KCG was public for a while.
Essentially, your trades have a lower cost basis than those of institutions. But the markets are generally priced assuming institutional traders. So if firms know you're not an exchange they can save some money on you, and Robinhood literally passes some of those savings to you.
(I don't love Robinhood or anything, but the model makes perfect sense.)
On a more meta level, my firm suspects that Citadel is trying very hard to price other players out of the market even if it's at the cost of their current profitability, so they might be driving up the terms of newly minted deals. I don't work on the retail desk so I don't know.