your tutorial is for version 1.14 / 1.15, but minecraft change the biome/world gen (also include entity spawning) in 1.16.2, if you try your code in 1.16.1 it will work
also you don't need to create a new Event, subscribe to it!
okay there you don't understand what to do:
use the code snippet from the static EventHandle and replace the event with BiomeLoadingEvent,
and then check out the methods in the event (e.g. BiomeLoadingEvent#getSpawns), than do your logic there