No, it wouldn't, because employers have a strong incentive to hide salary band violations, while employees have a strong incentive to discover when negotiations are possible.
> it strikes me as an extraordinary to claim successfully negotiating higher-than-average wages is typical.
Then you should probably go talk to somebody who made that claim. But it's a nonsensical claim, so my guess is not many would be making it.
I've never been a typical employee in my life. I have, however, been the highest-paid non-manager (or so I was told; this might have been an attempt to shame me into working more for free) in a hundred-person development group, well out of band, because I brought capabilities and skills to the table that they needed more than they needed to Maintain The Integrity Of The Salary Band.
And calling me "anonymous" is kind of funny.
I would also suggest your understanding of how often exceptions are made is very unlikely to be based on good data. Employers have ways of hiding that they are paying somebody more than their traditional salary structure when they need to make an exception. And a very strong incentive to do that hiding.