Earlier quoted context omitted.
Most of them aren't hedge funds, they're private (HFT, Electronic Trading, Botique Firms, Proprietary Trading are all terms you'll hear) so they don't have public data. It is literally impossible to prove without insider information. However the Virtu Financials, the Citadels, etc, will always exist, and will be doing exceptionally well whether people realize it or not.
I realize this doesn't actually matter and I feel weird defending Renaissance... but Virtu is a public company and hasn't been doing that well, and returns from Citadel's various funds are not hard to find. Also, I don't know how to compare a fund's returns to that of a private business. But Medallion has been around since before HFT was really a thing, and I'm not aware of any HFT places that have been growing at 70…
The issue is that these business are capital limited, so you can't reinvest your massive returns to compound them.