It is hard to believe that that number of people are still unaccounted for. It is likely many have left the area and are staying with friends or relatives and haven’t notified authorities, but sadly some could still be buried under the debris.
One of my kids lives in Tuscaloosa. She tells me that most of this statistic comes from family and friends reporting a person missing and when that person turns up, they don’t think to let the authorities know. I hope that is the case for the vast bulk of these people.