First of all, seat maps are not the most accurate way to judge availability. Expert Flyer is very useful when it comes to determining if a flight has seats.

Given conditions in ATL today, many flights are likely full or overbooked. No longer being for sale on DL.com is one indicator the flight is oversold. Empty seats on the seat map do not mean there's availability, as some pax get seat request cards and their seat at the gate.