Even if the prompts are technically leaked to the provider, how would they be identified as something worth optimizing for out of the millions of other prompts received?

Yes everyone is doing that on benchmarks but they are still somewhat useful and the likes arc agi even more, though we are not be able to quantize exactly how much better they are getting they are still necessary. For arc agi these are some big gains by which ever way the went about it, since everyone also has been trying to max it for the last 3 years but we do need to come up with better benchmarks/evals like arc tried.