I agree that the argument is adequate, and I am not suggesting it's false just because Euclid may have disagreed. That is absurd, and not something I believe. My point is that not all mathematicians have always accepted the principle of explosion as coherent -- or, at least, there is a sizable body of literature that looks at ways to get around it. I'm not trained in paraconsistency, so I'm not sure to what extent its logicians would agree or disagree with this proof in particular, but I think that being explicit is important. Justifying steps and clarifying assumptions is as important, in my view at least, as labeling units. But that's more of a pedagogical issue than a mathematical one.

I only mentioned losing half a mark because that's the classic way that graders note missing units. But if a comment or a circle suffices, then I would follow that.