is this rare, to have all these central pieces protecting each other and nothing else

Kind of "yes" and kind of "no." The strong chess players almost always have everything pretty well defended, or at least are aware of the ones that are not. However, this is not in the same way we have in the photo above, that looks a bit more uncommon to me. Usually strong players have pieces defended by pawns or other pieces of lesser value, but one needs to also be careful when doing this. 

Wonderful, thanks very much @KeSetoKaiba for this explanation, illuminating.

Yes all that tension that those knights had was tied up mainly just to rigidly protect each other eh, in that situation they were doing not very much else against bishop attacks except for pressuring that one g6 pawn.