No... it's the maximum weight (of anything) that the car is designed to handle and may not be exceeded.
8000 gallons of water is actually ~ 66,700 lbs.
8000 gallons of oil or gasoline for example is much lighter. Other products may be much heavier. 8000 gallons of corn syrup is ~ 87,500 lbs so if this car design is used to carry syrup it wouldn't be able to be loaded to the full 8000 gals.