In pure SU(n) 4D QCD, i.e. SU(n) Yang-Mills theory without matter, there exists stable long fluxtube strings which carry a 1-form symmetry charge. Over the past decade, there has been increasing evidence from lattice calculations that the worldsheet theories of such QCD strings contain a massive pseudoscalar (axion), at least when n>2. This has so far been puzzling from the perspective of holographic realizations of strings in confining gauge theories. In this talk, I will show how such axions appear naturally in the realization of 4D N=1 pure QCD from M-theory on non-compact G2 orbifolds. I will also show that this picture predicts that QCD string axions exist only if the gauge group is SU(n) (n>2), SO(4n+2), or E6 (or a quotient/double-cover thereof). I will discuss why I expect this pattern to hold for non-susy pure QCD as well which could be tested in future lattice studies of QCD strings for gauge groups of B, C, D, and E-type.