As others have commented there is NBBO( National Best Bid Offer) book that every broker has to respect. Price improvement that the article talks about is total

As others have commented there is NBBO( National Best Bid Offer) book that every broker has to respect. Price improvement that the article talks about is totally upto the broker if they want to pass it back to the customer or not. Or if they have a deal with their buyer. This happens a lot more in over the counter markets like FX. Overall Robinhood has been a net positive that disrupted & forced old retail investors trading platforms to go 0 commission