Not apparent at first, but this is a TypeScript to c++ compiler at the core (https://github.com/geastack/compiler), with bindings for various platforms.

What do they use for graphics and audio?

Ah this is for embedded apps. Pretty neat!

"The earlier implementation could not be driven to zero of them, because boxing was load-bearing in its emitter"

In the docs of the repo of their TS to C++ compiler. https://github.com/geastack/compiler