Your comment is glossing over the fact that high energy photons cause damage through a different mechanism not available to lower energy photons. A higher dose

Your comment is glossing over the fact that high energy photons cause damage through a different mechanism not available to lower energy photons. A higher dose of lower energy photons may cause damage still, but not in the same way Ionizing means that a photon has enough energy to eject an electron from an atom, ionizing it. This can disrupt chemical bonds, hence the direct damage to proteins, DNA, etc.