Engaging from a set distance isn't a rigged test. It's very realistic. It's the same as CAS aircraft loitering in the vicinity of fighting, not knowing where they'll be called to, then vectoring, approaching, engaging, and disengaging. The F-35 has an advantage in approach speed, and when you're under fire from the enemy, fast engagement becomes extremely important not only for your safety but for the safety of the CAS aircraft in that the enemy will have less time to bring in reinforcements.
I'm a huge fan of the A-10 and expect it will do better in a number of tests in a fair competition, including the raw number of targets that can be engaged primarily due to the gun's ammo capacity. I'm not a terribly big fan of the F-35, believing that it's trying to combine too many functions into one airplane. But for a CAS demonstration, I expect that the F-35 will be fitted with external stores (appropriate in an area where air supremacy and some level of major SAM suppression have been achieved). External stores capacity for the two planes is similar weight-wise (16,000 pounds on 11 hardpoints for the A-10 vs. 15,000 pounds on 6 hardpoints for the F-35), but a maximum load for either plane is unrealistic as it adds drag, reduces range and loiter time, impacts agility, and increases stress on the aircraft. A-10s in Afghanistan would rarely go out with more than a single weapon or pod per pylon, and often with four or more stations empty. The F-35, of course, also has the internal stores that can handle up to four Small Diameter Bombs each.
Low-altitude survival time probably goes to the A-10, but accuracy could be a toss up between the slower and more stable A-10 and the faster but perhaps quicker to lock and launch F-35. This may be a closer competition than many believe.