It would be trivial for them to opt-out accounts that are doing benchmarking from a planned regression.

I really really really do not trust companies on the verge of IPOing from goosing their numbers. Even if the lawyers are up their ass, the social pressures are more than enough to compel the behavior. For every good person willing to do the right thing even at the cost of a delayed career progression, there are 20 people willing to do anything not even for a dollar but for an "atta boy!" from the boss.

If it's a private benchmark, how would they know it's a benchmark?

That's a major reason to use local models. Or open weights models on a trusted cloud provider. Even if they are way less capable, they're more predictable