I'm not sure to understand the fix: Aren't caves supposed to contain different kind of rocks, ie. different minerals?

The problem is that 317 cave models do not have the correct textures assigned to them. If cave models are called 'lava' rock then they should have lava textures assigned to them and should match the other lava models.
What Bethesda did was to name models with the type of rock they were supposed to be then assigned a completely different texture to many of them so that none of the cavesets have rocks that match.

Maybe the idea was to have contrasting rocks and with Bethesda's textures it doesn't matter as their textures are similar in appearance. But when modders make texture replacers the texture they use will resemble the description eg the bone cave textures will looik like bone , the pyrite cave textures will look like pyrite etc so the contrasts will probably not look good for anyone using texture replacers.

So by correcting the models this gives a few advantages, for anyone using texture replacers the caves will most probably look better and more plausible because most of the rocks inside a cave will have broken off the walls and roof and therefore should look and be the same type as the walls , but using the original models means that you don't have any rocks that match the walls. Also modders making caves can better achieve the desired effects if the rocks match their descripotion. Modders can choose to have contrasting rocks from different cavesets but the result will probably not look right for many people using their mod.