Ergonomics/workplace safety officer here; you’re quite correct. The idea that sitting is the new smoking ignored the detail in the epidemiology: Inactivity is the real problem.
I actually teach my students about this strategy that the WHO employee in Micronesia in my sport nutrition class. It’s less about the iron fish, and more about that dietary iron can come from cast iron cooking sources instead of supplementation (as the latter often causes digestive distress).