But even if it were grill temp the gauge on the BBQ lid is more of a guide than an actual temperature. They are highly inaccurate, more like "cold, warm, hot" than "400 degrees".
For most stuff just dialing in the temp comparatively is fine, so the grill gauge works. You know you want to grill your steak when the thing reads 600 degrees, and your chicken at 400. But those temps are certainly not remotely accurate.
For some other things (e.g. BBQ/smokers/etc.) getting exact temps correct is key. In those cases you'll require something much more accurate than the grill gauge even just to measure grill temp. Having something wireless is pretty handy in this case, so you can watch a movie in the basement and check on your meat temps without walking upstairs every 15 minutes.