Xenon, if you insist on a binary definition of "cheating" (either yes or no), then you are forcing the conversation to be about where exactly to draw a fixed, rigid line. Probably the only place to justifiably draw such a line is in the letter of the IAAF rules (which appears to be the NOP approach in my opinion).
On the other hand, you could ask "How ethical is it", "how fair is it", "is there any moral issue with" this person taking iron supplements. Personally I'd say no. I think there is some ethical issue with hypothyroid medication unless there seems like a very fair medically convincing case that the person is "abnormal" and that this hurts their performance if they are "normal".
The interesting difference, as you identified, is the question of if more always helps as opposed to the dichotomy of "normal" vs "abnormal". I think it's sketchy to supplement on something where more is generally better beyond the "normal" range. But I don't know what specifically falls into such categories... Anyway, I wouldn't consider it "cheating", but I wouldn't want to do it myself.