Peeking at the source code, it looks like the biomes use the vanilla Biome registry (Biome.registerBiome) but not the Forge registry (GameRegistry.register). They should use (only) the Forge registry in order to facilitate compatibility with other mods (i.e. preventing biome ID conflicts), and so that the biome IDs do not have to be configured (since by registering using Forge, it handles IDs in a similar way to block IDs).