For whose money will the exchange exist? Who will pay for the equipment, ervers. It also requires high costs for sacurity and personnel. Most likely it's just a marketing move to attract new customers. But after a while a fee will be introduced and the exchange will turn into the usual one.
Robin hood already has a large infrastructure built out around the current trading platform for stocks. I am sure if they offer a mobile exchange that is stable, They will have a massive amount of people flocking to trade crypto, Especially from their phone.