It has a help text of "If disabled, it will spawn as any other nest", and the double negative (disabling the option to disable the check) leaves me a bit confused about what the option actually does.

Also I've noticed lots of large clusters of nests are just spitter and snapper nests, with biter nests almost exclusively in expansion nests. For context I'm using SE + KS2.

I've done some quick test in the map editor and it appears when "Disable moisture check on enemy spawning" is enabled, no biter nests spawn at all, seemingly replaced entirely by snapper nests mixed in with the spitter nests. When I have "Disable moisture check on enemy spawning" disabled, then biter nests spawn as usual, but in some biomes there is a mixture of snapper + biter + spitter nests, while in other biomes there is only biter + spitter nests.

Personally I would expect disabling the moisture check to spawn all three of snapper + biter + spitter nests in all biomes instead of replacing all biter nests with snapper nests.

I think I figured out why it is replacing all biter spawners entirely. It is because you simply copy the biter spawner's autoplace instead of generating a new autoplace. When enemy_spawner_autoplace is called it generates a random seed each time, but since you just copy the biter autoplace it generates in the exact same spots as the biter spawners hence the lack of biter spawners. I'll do some tests to verify that this works and then create a PR if it indeed fixes the issue.

Thanks for finding the issue and taking the time to solve it! I've updated the mod and reworked the settings like you suggested, to make it less confusing.