Because HammUEr imports uncompiled map files (like VMF) and not the compiled BSP, the geometry you get out of it for a given map is not what you'd see ingame for that map, because the compile tools on the old-engine end (eg VBSP) do a lot of work to convert that vague data from the VMF into quite a nice mesh, eg: culling interior faces.
On UE4 this generally meant it was advisable to spend a bunch of time NODRAWing faces that you didn't want importing into Unreal, but as it happens in UE5, Nanite and Lumen means HammUEr's relatively raw import is actually preferable to the post-VBSP result in most cases because you'll get better distance fields and cards for Lumen with those extra unseen faces, and Nanite means you don't give a shit about all these extra tris and draw calls (to whatever extent you did anyway).
Cool!