I had a 2020 skidoo tundra 550f in the back. The machine is 480lbs with a tank of gas. Most of the weight of the machine is on the front up by the cab, not at the back of the box lol
My atv was a polaris sportsman 500ho. Maybe 600 lbs? Again, not overly heavy.
So i'm my opinion, no, not overloaded at all and you should be able to load things into the box of a truck including ATVs and snowmobiles. Hell even the owners manual shows the use of ramps for loading wheeled vehicles.
Out of curiosity when I went and got my new truck, I started looking at other used chevy trucks on the gm/ chevy dealerships in town. Noticed a few other tailgates with the stress cracks at the top left and right corners as well.
Just something for people to keep an eye on, thats all. I know I won't use the bed of the new truck like the old one. Picked up a utility trailer instead lol