Earlier quoted context omitted.
Zero commission trade is actually bullshit anyway. You end up losing more from inferior execution time than you would if you just paid the $5 per trade. Robin Hood also lied to users about this and ended up paying a $65 million fine[0]. [0] https://www.sec.gov/news/press-release/2020-321
How is this possible when I use limit orders? (not a robinhood user btw, but pretend I am)
You place a limit buy order at $10.55 for 100 shares.
$0 commission broker that sells order flow: Your order is routed to the market maker buying your broker's order flow. They sell you the 100 shares for $10.55 since it's within your limit and within the NBBO spread.
$5 commission broker: Your broker attempts to price improve by searching multiple liquidity sources and gets a hit at the NBBO midpoint: $10.50.
In both cases you paid $5 for the trade.