What makes Bangle.js different then other smart watches using Espruino

implements some of it,

Well the idea is to implement as much as possible and then maybe provide stubs for parts than are not relevant - like missing hardware (compass, gps) or missing implementation (HR sensor)

I think the implementation completeness was simply driven by apps that people wanted to run on their watch. so @user156811 you can definitely improve it and add parts that are missing for apps you want to use or try to make jswrap_yourwatch.c in C or even try to hack your changes into real jswrap_bangle.c which IMO is hard to do and also hard to maintain over time.