Note that after the pilots said that they could go visual the preliminary report documents the following

> The ARTCC controller subsequently cleared the flight to SRR, cleared the flight for a visual approach, and informed them they could cancel IFR in the air above 9,000 ft msl or they could cancel their clearance via the flight service station after landing. The flight crew acknowledged the clearance and responded that they would cancel IFR “in just a couple of minutes.” There were no additional radio transmissions received from the flight crew.

It's not at all clear to me that they did go visual. They communicated that they intended to cancel instrument clearance later and that suggests they were still using it...