Yeah, the special graphics levels' level data are exactly like that also in the original games (DOS, Amiga, etc.) They contain no terrain because the terrain comes from a separate file that is basically a direct bitmap image of the terrain graphics, rather than building it from terrain pieces like regular levels. However, I don't know if these separate graphics files, or the associated programming code to use them, are also ported to WinLemm or not. I would say it's less likely.
The fact that the level files are all included is simply because they can be easily reused from port to port with no modifications. The level files basically just specify the positions of the terrain pieces and objects in the level (and things like which graphics set to use), and otherwise contain nothing like actual graphics that would need to be handled differently from port to port. So most likely they just mass-include the entire set of level files from the original game, and then because they are so small, there's no need to try to keep the level files in sync with which levels are actually exposed normally to the user in WinLemm.
One quick test you can try is to simply replace one of the levels exposed in the game with those files (just like you did with "All the 6s"), and see what you get trying to play it in WinLemm. Most likely you'll just get the entrance and exit without the special graphics terrain, which would indicate that the support for special graphics is simply not included in WinLemm. (Granted, it could also be included but disabled in the programming, unlikely as it may be.)