Do they generate js/web app from Haxe and run it inside the bundled Electron? Why?

Yes, the code base is Haxe and it "transpiles" to JS + node + electron. The idea is I'm more comfortable with Haxe but I really wanted to have a ~somewhat~ future-proof technology for the app (ie. JS/HTML5/CSS)

If people complain about the binary size and/or memory usage, one tip would be to see if it'll run with either NW.js or Tauri, should be close to a drop-in replacement.

Tauri is even smaller than NW.js (I think ~600kb hello-world binary size) but uses Rust in it's toolchain way under the hood to build, so take that as either a positive or a negative.

https://github.com/tauri-apps/tauri

NW.js is essentially lighter-weight Electron.

https://github.com/nwjs/nw.js

Would be neat if it was a straighforward implementation and produced tiny self-containing cross-platform binaries.

Disclaimer: I also have no idea how gamedev works though, so this might not be applicable