Cave by definition has roof and your pic shows only one cave on the right.
I guess you missed the part where I stated: "Perhaps not so easy to see in the photo that I posted, but even in the two middle boulders, there was enough for a large adult jacob peacock to utilize as a cave." There was also more than one cave in the pile on the right, but again, you aren't seeing the complete set up so that may not be obvious.
You are only seeing a still image, from one angle.
And as already stated, the vast majority of fish in that tank were indeed, open water species, not cave dwellers. When I kept Tangs, and mbuna, I had multiple caves, all created using river rock. No plants found where most of those species range, so none needed in my tank for diversity. lol
Ditto to some of the open water spawning species, or egg scatterers such as some of the Rift Lake Synodontis species. I had no issue breeding them, either.

For Alto comps, I created caves as you described using slate set up like a flat walled teepee - but no matter the species, and I kept many over the years, the end result was always a fairly natural looking scape, not one made out of rainbow rock or lava. But as I said previously, to each their own. I'm not pointing fingers, I currently have a tank with ABS pipe in it, to provide escape routes and hiding spots. Far from au natural, but it works.