Indeed, the premise is just wrong. It's not as if organisms cannot process biomolecules of the other handedness. There are enzymes called 'epimerases' and 'race

Indeed, the premise is just wrong. It's not as if organisms cannot process biomolecules of the other handedness. There are enzymes called 'epimerases' and 'racemases' (https://en.wikipedia.org/wiki/Epimerase_and_racemase https://en.wikipedia.org/wiki/Epimerase_and_racemase) that convert stereocenters. While it's true that a mirror cell wall would be a barrier to being broken down, there's nothing that would fundamentally stop the evolution of a suitable degradation machinery for it.