Look at the companies listed in the disclosures. Citadel appears to have the largest share in all of the examples listed. Their website [1] claims that:
> We act as a specialist or market maker in more than 3,000 U.S. listed-options names, representing 99% of traded volume, and rank as a top liquidity provider on the major U.S. options exchanges, executing approximately 39% of all U.S.-listed retail volume, making us the industry’s top wholesale market maker.
Citadel is paying Robinhood to put your limit order into their system at the NYSE or NASDAQ or whatever other exchange and then they handle the execution of it. Also, I don't believe that this article is claiming that Citadel et al are the high frequency traders, but that the high frequency traders are connected to their platforms and profiting (in some unspecified manner) from the orders that come in.
[1] https://www.citadelsecurities.com/products/equities-and-opti...
...OTOH, what that product revealed was that Reg NMS violations were pretty rampant and the SEC wasn't interested in doing anything about it. You may have difficulty enforcing this.
Orders take time to process. HFT guys are fast enough to react and make money from it.
Say the price of the stock is $101. You place a limit order for $100. HFT sees that but can’t do anything since the stock price is above the limit.
Stock price drops to $99.5. Your order is the executed by the exchange before the HFT can buy at $99.5 and sell to you at $100.