Factorio Forums

In the mod settings:
Start up > Use vanilla biter generation (unchecked by default)
Map > Use RSO biter generation (checked by default)

This is a bit confusing... they counter each other and checking one does not disable the other.

First one is about vanilla biter generation.
It needs to be in startup tab since it affects data stage of game loading.

Second one is enemy generation by RSO itself and can be toggled in run time since it controls mod code directly.

Both of them can be enabled which leads to enemies created in both ways.