Yes and no. The Unreal approach makes it easy to create levels with a waterproof outer hull which allows you to use certain optimizations (such as portals in t

Yes and no. The Unreal approach makes it easy to create levels with a waterproof outer hull which allows you to use certain optimizations (such as portals in the BSP tree) to reduce what you need to draw. But even without that, CSG usually produces only geometry that has a thickness, and that's what helps with the sliding through floors in the first place. But since the Unreal approach makes it very easy to query if a point is inside the valid space of the level, or not, that's also used as a shortcut for determining if things did fall through somewhere. So yes, the "blank filled with mass" approach further reduces the problem but no, it's not strictly necessary.