Playing around with the most recent code in your Fabric-1.17.X branch and noticed this in byg-world.toml:

	#What biome(s) does Therium spawn in? Default: "byg#THE_END".
	theriumSpawns = "byg#THEEND"

Looks like the default value (THEEND) doesn't match the what the documentation claims is default (THE_END).