>thinking one more hour would put them right in the 203 tender zone.
Not sure what tender zone means. Meat gets tender when held at temp over TIME. The higher the temp, the faster the connective tissue breaks down and they get tender. If you cook really hot, they may reach 210F and still not be tender cause they haven't cooked long enough. Conversely, if you cook at 150F for long enough, they'll fall apart. Same is true for brisket, pork shoulder, etc.
>>It has to be the cooling effect of evaporation taking out of foil and ribs being thin.
Absolutely, lots of surface area and not a lot of depth.