The source map is there. The whole thing is made with JavaScript and with very little (pretty much no?) dependencies. No React, No TypeScript, No 54.643 packages to download. He seems to use lesscss but that's about it.

Sometimes I wonder if we lost the art-craft in all of this frameworks mania. This work has better performance than 99.9% of the apps out there despite being reasonably complex (UI-wise). I legit though this was built with WebAssembly at the initial interactions.