The way the DEA does it, every plant counts as 2lbs no matter what stage of growth it's in. they don't do any weighing unless it's already harvested. At least that's how they did it for me.
Looks like you're spot-on, 1000 plants, 2000 pounds, the dumb thugs just multiplied by 2. I'll bet most of the plants were tiny cuttings, no way you're getting 1,000 plants under 10 HPS lamps.