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)
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.
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