What you’re looking for is DataQuery.of("customSettings")
I investigated generatorSettings and found that an issue was reported some time ago:
So I went ahead and fixed it
The following code now works:
WorldProperties worldProperties = Sponge.getServer().createWorldProperties("flatWorld",
.build("myworldarchetype", "My World Archetype"));
WorldSettings is an internal minecraft class that is similar in functionality to Sponge’s WorldArchetype, and likewise WorldInfo is similar to WorldProperties
They both have a generator setting, but WorldArchetype is used as a template for creating worlds whereas WorldProperties is a collection of properties per world instance.