If you have Exclude Selected Tiles enabled on your cartographic polygon, then it will exclude tiles that fall entirely within the region. But any tiles that are both inside / outside of the polygon must be loaded – otherwise, you’ll have holes outside of the polygon.
For those polygons, a material effect is used to hide the parts of the tiles that fall inside of the tile, leading to the appearance of a crisp region. However, the original meshes of the tiles will still be there. That’s why the tutorial is called “hiding Parts of Tilesets” – the clipping is intended as a visual effect. It is currently not possible to have the polygons actually cut parts of the tile meshes as they load in.
I hope that explanation makes sense. Let us know if you have any follow-up questions or concerns!