I kind of (sort of) get it. In 2015, I took my WRX to the Tail of the Dragon and the surrounding area (Moonshiner 28, Cherahola Skyway, etc.). The WRX was very much at home on these roads. Smaller size and weight allowed for a bit of fun at speeds that didn't threaten my license.
In 2017, I took my Camaro SS to the Tail and Moonshiner 28. The SS required a lot more diligence. Yes, it built speed like no WRX and its handling limits were very high. However, one incident of less than 100% attention led to a both feet in event. It was totally my fault for playing around with some sport bikes. I stuck the front end into the corner and late trail braking was planned. However, I was suddenly in a drifting mode due to a bit more movement of inertia. One way out. Both feet in and may the gods of technology bail me out. Really good nannies and a few hundred track laps of muscle memory showed up at the right time. You would have had to have been some kind of extra special stupid to get into that shape with the WRX. With the WRX, you could explore your limits and the cars limits with a larger margin of error.
With 760HP and 700+ pound feet; coupled with ~4,200-4,300 pounds of heft, you would need to be constantly on your game to prevent some type of event. I think the STI would be easier to drive closer to the limits of those roads in Northern California. Its called a momentum car on the track. I can see where that would keep the STI closer to the GT500.
Fine. Even if the S209 outhandles the GT500... IF it does. It still has only 50% the horsepower. Yes, it depends on the track, but I can't think of any sanctioned event outside AutoX that this type of power:weigh discrepancy can be overcome by svelte athleticism.
I think it likely that the GT500 wasn't blitzing each and every straight. Maybe a bit to catch up or put a bit of distance between them, but not full bore for all of the straights. I think in that scenario, the GT500 wouldn't be running away and hiding.
Airbags, ABS, traction control, 200 speaker sound systems, etc. all adds up. Porsche 911s are ~3,200 lbs or more. Add AWD and you add another ~100-150 lbs.