> here's no outside investors in the Medallion fund anymore, so if it is a scam, they would only be scamming their own employees

The "it's a scam" theory that I heard had nothing to do with it being a Ponzi scheme, instead it was about laundering high-tax income into low-tax capital gains while maintaining a plausibly deniable investment cover story. It didn't have to beat the market, it just had to beat regulators.

I have nowhere near enough domain competence to comment on the plausibility of that scenario, other than to say that I suspect if it were that easy everyone would be doing it, so on those grounds alone I suspect the conspiracy theory I heard was incorrect. Someone more invested in the idea that it's a conspiracy would likely just argue that it isn't easy to be good at this or that there are economies of scale, so only one dominant player. Shrug.