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My hunch is that the precomputed visibility data (i.e. previs) is a different method of occlusion culling. The official documents seem to imply that the previs system only applies to preplaced static geometry, so dynamic objects and objects added by the player in workshop mode should not culled.
The occlusion planes I see with STS appear to work differently, since they seem to cull almost everything. It’s possible the vanilla game includes no occlusion planes of this kind.