I set both
#Spawn weight of Rainbow Eucalyptus (treemendous:rainbow_eucalyptus_forest) biome (higher number means more frequent)
rainbow_eucalyptus_forest_weight = 0
#Spawn weight of Rainbow Eucalyptus (treemendous:rainbow_eucalyptus_forest_hills) biome (higher number means more frequent)
rainbow_eucalyptus_forest_hills_weight = 0

to 0 and the biome still spawns.

I think the weight system for biomes doesn't support 0 weight, I'd have to check that. But I'll look into this when I find the time.

The new 1.17.1 version fixes this issue, skipping generation of a biome if its weight is 0.