That is true, but nVidia's outreach engineers have a history of checking code that regresses performance on competitor hardware. See what this Value developer has to say about "Vendor A":
Vendor A is also jokingly known as the "Graphics Mafia". Be very careful if a dev from Vendor A gets embedded into your team. These guys are serious business.
So, you are suggesting that game studios let vendors check in code totally unreviewed? I worked at a company that had two engineers from Nvidia and 3 from AMD - none of them had the ability to check in code, although they did have access to our sources.
The Nvidia engineers were top notch, knew their products, knew how to get performance from their products, and would be unhappy if we didn't take notice of what they said. The AMD people were OK, but just not in the same league as the Nvidia people. Which was best for us? The Nvidia guys improved our product for both their customers and AMD customers. The AMD people would only look at AMD specific code and provided way less assistance. I'd go with the Nvidia guys any day - they were indeed serious, hard working engineers, one with a ph.d., the other with a masters.
And the best you can suggest for fiddling is a benchmark from 13 years ago? That several lifetimes in graphics technology - go look at any 2001 game. As I recall, both Nvidia and ATI (as it was then) tweaked benchmarks to favor their product around that time and were found out. However, modern graphics benchmarks make it difficult for any manufacturer to corrupt the results.