Here's my calculation, getting punchy looking at it, but believe I am correct.

UTC is not an arbitrary system, it is a system of 24 time zones of 15 degrees each. Time being calculated as noon when the sun is it’s highest at the center, central meridian, of each zone.

Thus, the central meridian of the Eastern Time Zone, longitude 75° W, noon is the time the sun crosses it’s meridian, i.e., solar noon for that position.

The central meridian for the Eastern Time Zone is 75° W Longitude. Chattanooga is located at 85° W longitude.
The sun traverses 15°, one time zone, per hour. This makes a 10 minute difference in time between these two points.

Thus, if sunset at Chattanooga is stated as 7:42 EDT, actually 6:42 EST, it is actually 6:32 PM local, solar time.

Hence, the officer’s statement that it was “after dark” at 7 PM, would be correct.

"After dark" being rather arbitrary as to it's actual meaning.