Are there revese engineered open source version of the other games?

I assume you ask where in the Git mention the Flathub link since Flathub deems it unverified? It's on On Linux section of the ReadMe

I never tested it but I could find world2minetest or alternatively:

Alternative 1 - OpenStreetMap + Real Terrain Mod

You could download heightmap from OpenStreetMap to use Real Terrain mod to generate it.

Alternative 2 - Use whatever tool to generate for Minecraft then convert from Minecraft into Luanti

You could use mcimport or MC2MT(Luanti forums thread)