Nice looking beef. I never seem to find beef ribs in stores that are worth a damn. (I stopped cooking them when I got tired of waiting hours just to eat a scrap of meat.) But they're a favorite here, so the search continues.
I like the idea of the long stem thermometer in the silicone grommet. So long as the thermometer works properly I see no reason why that shouldn't provide an accurate pit temp.
My 18 WSM came to me with a large aftermarket lid thermo that always reads low. Like 100 degrees low. Not sure if it's just worn out, or if the situation is the opposite of a kettle's lid thermo, which normally reads high relative to grate temps.