Hey, thanks for filling my request!

However, when I try to load a superflat world, it still seems to generate as usual, at Y=-60, with grass blocks as far as the eye can see.

How do I get this to actually work?

EDIT: Ok, so I loaded up a default world with this datapack, and it loaded like I expected. However, there's another problem:

The aqua biomes don't seem to load properly.

I used a command to search for a mushroom fields biome. Those are always supposed to be islands surrounded by deep ocean. However, when I teleported to the biome, it was surrounded by grassy plains. When I pressed F3 to pull up the debug menu, it still said the biome was "deep ocean," but I was standing on solid ground.

My random seed is -6817391374897860619 and the mushroom field biome I am currently located in is at coordinates -1461, 63, 1413, if you want to check to see for yourself.

That is awesome!!! How many biomes did you get working?