If Hamas put on uniforms, the number would be much higher. The war would also have ended a long time ago -- and not in their favor.
That does not excuse Israel's own war crimes, which are indeed egregious. Israel has obligations, both moral and under treaty, that it is violating.
But I think it's important to consider the framing of this article. The implication is that Israel detaining exclusively "fighters" is easy, but in fact it's not possible.
I'd love to see a convention on asymmetric warfare, parallel to the Geneva Conventions. What are the obligations of a first-world country fighting a much smaller force who is not bound by the existing conventions? Obviously more than Israel is engaging, but I don't think most people would agree to fight solely along Geneva Convention lines, either, when faced with such an opponent.