The gut contains 70% of immune cells because your gut is actually the main point of contact between your body and the external environment. The surface area of your gut is somewhere from 20x to 100x that of your skin.

One could just as well argue that because the immune system is so well developed and robust in the gut that it's the last place to look for dysfunction. Or one could argue that a 70% share of immune cells is rather low considering the relative ratio of environmentally-exposed surface area.

We could say all kinds of things if we're free to make unsubstantiated conjectures. But we really can't say anything of substance without context and evidence. The human organism is ridiculously complex and consistently defies intuition.