Sorry I don't have much time to see the full video but what I saw in the first few minutes where he built a room, is nothing special, I've done the exact same workflow for example in Max Payne 2 editor.
What he means with mesh's, is that the geometry you work with inside that Unreal Engine plugin, is the same one you do when you work in say Blender, Maya, etc, is just triangles and quads. Meaning the geometry is not composed of simple mathematical planes like brush's but real 3D geometry.
Why you don't use portals in there? Because Unreal Engine 4/5 afaik have no portal system for visibility culling.
Portals are a CPU concept and are normally used to prevent to render things that the player doesn't see during gameplay, but is not the only way to do this job. I think ever since UE4? Epic's engine uses a more advanced system (but more expensive one) called GPU occlusion queries, in simplistic broad terms, it uses the GPU to do "porteling" automatically for you. btw Unreal Engine also used CPU based, manually placed portals not that far in the past.
Also the idTech4 BFG fork RBDoom3BFG engine also supports occlusion queries but on the CPU, using a Intel made library, perfect for open spaces.