I think they mean "on demand" is that the Cas9 proteins is activated upon activation of the light source. As the CRISPR/Cas9 complex is already in place due to the chemically boxed guide RNA, the way I understand it from briefly skimming this abstract is literally that you turn on the light when you want DNA cleavage, so I don't see anything weird about it, but maybe someone with more insight can correct me if I am wrong.