Trump was arrested. An arrest follows an indictment in every case. They just decided that it wasn't necessary to fingerprint, mug shot, perp walk. New York state law says that every non-violent felony offender is released on their own recognizance unless they're a flight risk. Which, Trump obviously isn't because he has a 24/7 secret service detail. It was always going to happen that he was released after the arraignment, until the next court date. That's what happens after every arraignment where there's no bail set. There is no arraignment without an arrest.