Maybe someone can devise a distributed bench-marking system where multiple people collaborate on tests and also vet each other's tests and rating without revealing them to the public.
I have my own "interview questions" for models where I give them a premade Git repo and a problem to solve. Then, I rate them like a teacher. I believe other do that as well, so we only need a reliable system to aggregate these results.
The problem with proprietary models behind APIs is that they could have saved your benchmark for future training though.
The only way to make it fair is to have the model provider give some benchmarking org the weights + inference engine, so that the model can be run in complete isolation and no information about the benchmark is leaked.
Though I guess for a 'random' person's benchmark that hides between all other requests it's probably ok.