They are, but the Supers are still kinda heavy relative to other race cars. The Supercars have a minimum weight of 1335Kg, or a little over 2940lbs. By comparison, and depending on the car in question, GT3 cars usually have a
maximum weight of 1300 Kilos, or 2866lbs. As I understand it, Supercars also have more aero to work with and are overall more maneuverable than current NASCAR Cup Cars, even though they have skinnier tires than Cup Cars (300mm on Supercars vs. 365mm on current Cup Cars).
So, SVG took a car that, while having better potential mechanical grip, had less aero, less maneuverability, more weight, and was able to not only pump out faster laps than current NASCAR veterans, but outright win, on a wet track, while having pretty much zero significant experience with the Cup Cars, while almost everyone else was suffering from brake fade/failure and low traction for most of the race.
Also, very minor note, but I loved whichever camera they had set up on the fence that shook like crazy everytime the field passed by. I have a feeling it was totally by accident, but it was a really cool accident that did a great job showing the power of the Cup Cars thundering by.