If you read the article, the answer is iron (and all the other elements) did indeed oxidize, but with water, not with free oxygen in the atmosphere. At least, that's what the experiment says could have happened. The reason for this is chlorates and bromates in the water that enable this reaction, at least in the case of manganese. Whether or not this applies to iron I don't know. What is important is that manganese doesn't turn into manganese oxide when there is oxygen in the air if you also have too much CO2, so unless your model has the Martian atmosphere "just right" to make manganese oxide, you need an alternative process.