You know, that is an important clarification, thank you.
The water content in the rust and what that means for the formation conditions are really important and exciting!
I was trying to emphasize that the iron oxide part has been known for millennia. The technique of trying to compare materials based on their color is older than civilization, though our methods have gotten a bit more precise in recent centuries. I was trying to humanize the act of spectroscopy into something most people were familiar with. In doing so, I left out the discovery of water that made this new research so exciting.
The fact that there is also plenty of water in the dust is an important new discovery, both for understanding how the dust formed and for understanding the current water budgets and cycles on Mars. I wonder how easily that water gets exchanged between the dust, atmosphere, and subterranean reservoirs. I wonder how the dynamic distribution of water interacts with Martian weather, including its global dust storms and the large seasonal atmospheric pressure variations.
Mars’s iron content is not that far off from the Earth’s mantle. The name for this type of rock on Earth is “mafic”, because of the magnesium and ferrous (iron) content.
On Earth, the processing of plate tectonics selects for iron-poor rocks within continental crust in favour of felsic (feldspars) silicates with a lower melting temperature.
Mars has no plate tectonics and therefore has a more primordial mafic composition.