The impressive part is how big modding is in minecraft despite Mojang never providing a mod interface, I don't know it may be different now but the general route for a long time was decompile the java byte code make your mod and recompile. There were a number of heroic teams that made mod interfaces out of this but a whole lot of nothing from the actual devs. I came off the quake modding scene and was a bit horrified by the whole situation, with quake they gave you the game code, map compilers and later the source to the engine.