Even without knowing the full details, their returns are quite plausible if you have less speculative knowledge of their business. The challenge for most hedge funds is that any "edge" they find decays over time, often quickly, which leaves them scrambling to find a new edge that will allow them to outperform the market. Most hedge funds go to a lot of effort to preserve an edge to the extent they can.

Renaissance focused on finding and developing novel mathematics that can be exploited to find an edge, some of which is extremely esoteric and unique, and then massively automating that. Consequently, they can find more new edges before breakfast than some hedge funds find all year. The real effect is that they don't need to milk a dying edge for returns because they are constantly generating new ones, and they can pick the best of the bunch at any point in time. The investment side of the business is mechanical. Their recruiting has reflected their deep interest in novel theoretical approaches to edge discovery.

In a sense, they are a higher order abstraction of a more typical hedge fund.