This is incorrect. To understand the business model you first have to understand the order book and how trades normally work. The order book consists of two sides. The bids (people who want to buy, how much and at what price), and the asks(people who want to sell, how much and at what price).

For instance the bid side may say

Alice wants to buy 50 shares at $20 (order added at 10:37)

Bob wants to buy 30 shares at $20.1 (order added at 11:18)

Citadel wants to buy 35 shares at $20.1 (order added at 11:17)

David buy 10 shares at $20.1 (order added at 11:01)

The bids are ordered by price, and ties are broken by who placed the order first.

If Evelyn (Robinhood customer) shows up and wants to sell 5 shares at $20, then the exchange will say, oh nice 20 is less than $20.1, lets get some trades going! $20.1 is the highest prices, and of those orders David's were entered first. So Evelyn's sell order is matched with Davids buy order. The price is determined by the order which was in the book. So Evelyn will get a better price than she hoped for!

If Evelyn had requested to sell 100 shares, then she will first sell to David for $20.1, then Citadel's for $20.1, then Bob's for $20.1 and finally some of Alice's shares for $20.

Ok but what happens if Citadel is paying for order flow?? Now the order book (from Evelyn's perspective) looks like

Alice wants to buy 50 shares at $20 (order added at 10:37)

Bob wants to buy 30 shares at $20.1 (order added at 11:18)

David buy 10 shares at $20.1 (order added at 11:01)

Citadel wants to buy 35 shares at $20.1 (order added at 11:17)

So if Evelyn (Robinhood customer) shows up, and again wants to sell 5 shares at $20, then she will sell to Citadel, even though their order is later than David's.

So to sum it up

Evelyn (Robinhood customer) is unaffected

Citadel wins

David loses.