Yes, that number accounts for a 150lb person in every seat in the truck. The yellow sticker on the driver's door or door frame gives you the full payload capacity of the truck, if the truck is new enough.
Now, forget about this whole LEGAL business. There is nothing legally binding about any of those numbers in any state in the United States when it comes to private vehicles and RVs. You can register your truck for whatever you expect it to weigh.
If you take your truck to the weigh station, with an empty bed it will weigh around 2800lbs on the rear axle. The RAWR is 6084lbs. That means you have almost 3300lbs of payload capacity on the rear axle before you run into the first limit, which is the tires.
There are people who haul 4000lb campers on their 2500HD trucks, with no issues. They are of course heavily modified, running 19.5" commercial truck tires and suspension add-ons like airbags. I'm not sure I'd go that far myself, but it has been done.
2500lbs is well within the comfortable capability of a 2500HD pickup truck.