They haven't taken outside investments for many years because of capacity constraints with their strategy. It goes beyond that actually: they force people to take dividends of their profits, because the strategy can only handle so much capital before marginal returns plunge-- otherwise they would quickly have too much capital if they kept reinvesting their cumulative profits.
I thought the strategies varied over the years: early adopter by jumping on different bleeding edge strategies. Strategies restricted by limited fund size.
To add: “So, how does the Medallion Fund make money? It finds individual patterns in data and exploits each pattern just enough to turn a small profit. And when you add up all of those small profits, you end up making a lot of money.”