The biome_source properties for the vanilla dimensions in the world preset json file are set to a simple single-biome source right now. They could probably use better sources, maybe even the same ones used in the regular worldtype see here

Random thought - is there some way to add TerraBlender compat such that the biome is whatever TB decides it should be?

Yes please! as of right now it seems to be only plains in the overworld, and only nether wastes in the nether which means no piglins