According to their site, they claim that you'll "buy and sell at the best possible price"; I'm not sure how they are able to make this claim if they cannot route your orders directly to the exchanges.
On their mockup on their site, the orders screen has an "Estimated Price" which is a strange field as typical order types should be either "Limit" or "Market" and should show the bid/ask spread on the current security; so I'm guessing that the best "possible" price is the "market price" and they are doing a combination of
1) Internal matching; orders aren't being routed to exchanges but if are internally matchable, just reconcile the books internally and they collect the bid/ask spread on both side of the trade.
2) Payment for order flow: they route your orders first to Citadel and Timberhill where marketmakers love to take the other side of retail order flow.
3) Internal marketmaking: they'll take the other side of your trades themselves and try to immediately close their negative positions by routing the order to an exchange for rebates and a better price.