It's not anything like an api... it's like saying the command blocks were a step towards the api.
All it is, is a neat way for the devs to add small additions easier and have the same capabilities as mods that let you add basic blocks or mobs via json.
But you cannot add anything beyond the limits of a simple text file to minecraft with this.
Forge as an third-party sort-of-api gaves you amazing things like Thaumcraft and EU/RF, Moving entire villages with Frame-Machines... this "api" would allow you to add a different variant of andersite.