First, the dealer or manufacturer should respond to this question. You should clearly state your intentions and get the dealer to add a statement to the bill of sale that the unit will carry a toy of XXX number of pounds safely.
Since the unit has a load capacity of over 1600 pounds, it seems reasonable that a toy hauler with that much load capacity could easily handle a 500# toy. That, however, is a pretty light ATV. Be sure about the toy's weight. My big road bike weighs closer to 650#.
The floorplan suggests that the fresh, gray, and black tanks may well be forward of the axle. If that's the case, the fresh tank capacity is about 350 pounds of water, and that will help balance your toy load. Also consider that the pass-through storage under the front bed may be filled with several hundred pounds, including a 2K generator, and, perhaps fuel for the toy. You can check the location of the fresh/gray/black tanks. Bear in mind, however, if you drain the fresh tank boondocking, then you visit a dump station far from home, that ballast will be gone and it will alter the weight and balance considerably.
If you were to load nothing but an ATV in the trailer, you'd probably come close to reducing the tongue weight to zero. But if you think of a typical "balanced" load and the way the trailer encourages you to load up for a weekend, it's likely you'll balance the ATV with all the "stuff" you pack into the trailer in front of the axle.
It's also reasonable to expect that the trailer, designed as it is to carry a large toy, can handle the rear-biased weight reasonably well. The axle may be mounted with a rear bias in anticipation of a garage full of toys.
All up, you unit will put close to 5000# on that single axle....and more importantly on those two tires which are likely to be off-brand Chinese tires. I suggest you go to Harbor Freight and get an electronic infra-red thermometer and check them often when the trailer is loaded and traveling at highway speeds. What you're looking for is more or less equal temperatures for both tires. While you're at it, check the temp of the wheel bearings. A blow-out with a tail-heavy trailer could be the thrill of a lifetime.
You are wise to ask...and investigate. After you buy the trailer, I think it would be a good idea to hook up to your tow vehicle, load the trailer as you'd typically plan to (perhaps with a borrowed ATV), then go to a public scale. Check your weights while hooked up, then back the axle off the scale and disconnect so that only the tongue jack is on the scale. If you're in the 400# to 500# range for a typical trip, you'll be in good shape. (I think the factory spec tongue weight is a bit over 400#) And, with the other weights, you'll know for sure that your typical load is within the limits for the trailer's GVWR. Finally, you'll know if you're within the TV's GCVWR. A scale test will give you quite a bit of peace of mind.