In addition I have complex, dynamic geometry to render. And I wrote a custom, fixed-precision integer math library to prevent rounding errors for the almost infinite game world. Other products have weird rounding errors near the start, the middle or end of their worlds because of inconsistent floating-point accuracy. Moving to doubles only mitigates the problem instead of solving it.
And then it's just efficiency too. I have no idea how or why Unity or Unreal will treat my assets. As demonstrated on the stream: the game instantly switches between visual styles and texture quality. It lazily loads all resources so the main thread is only stalled when doing bulk uploads to the GPU. This makes the game start up in less than a second, something I couldn't pull off with a COTS engine.
Let me know if this answer didn't address your question fully.