First, I believe that number comes from this article[0], which doesn't tell the whole story. Did some research and found some more info with the help of this article[1]. In Q1, Citadel paid $12,583,220.60 to fulfill orders from Robin Hood[2]. I can't put together the total percentage of their order fulfillment revenue, but this website seems to suggest 91 million[3], but if you add up the totals from[2] they don't sum to 91 million--so I can't quite determine the percentage of revenue it works out to, but it appears Citadel is their biggest order fulfillment client.
[0] https://www.bloomberg.com/news/articles/2018-10-15/robinhood...
[1] https://fortune.com/2020/07/08/robinhood-makes-millions-sell...
[2] https://cdn.robinhood.com/assets/robinhood/legal/RHS%20SEC%2...
[3] https://www.businessofapps.com/data/robinhood-statistics/