I'm not suggesting anyone is doing anything, just stating the objective fact that it is definitely possible for closed-weight model developers, and would be super hard to detect outside of this limit scenario you posit, where it is provably impossible for the provider to have seen the benchmark before it was run (which of course would mean that the benchmark was created entirely "by hand" or using some other provider that is unconnected to the provider you are benchmarking).

To put it another way: a closed-weight model is, by definition, impossible to independently benchmark.