My $95 Thermapen gives me instant readings from the very tip. A typical use, I'll be pan frying a piece of fish or grilling a burger, I slide the tip through the food and watch the movie as the temperature changes.
Ask the GGG people how their thermometers work. What zones along the probe contribute equally to the reading? If they're as sensitive at the tip as Thermapen, you're fine.
And as I've said before, it can be off if the error is consistent. We just need to learn to cook with it. Most of human history, people have relied on instruments with unusual response curves. No instrument has to be "right". We want instruments that can help us to react, that can predictably tell us what to do.
You could establish a stable fire, and swap TelTrue and GGG thermometers every ten minutes. Is there a difference? Is it anywhere near the difference between measuring by the grill and measuring by the dome? One cooks relative to how one measures.
Let's say that measuring just inside the dome doesn't correlate perfectly with measuring Tel-Tru depth inside the dome. We're talking a very complex system here. Would this mean that measuring just inside the dome is broken? Or has an utterly fascinating rabbit hole just opened up before us? We want the dome material itself to heat up, we want it to radiate heat... if the dome is lagging behind pit air temperature, that tells us something about where we are in our cook. Near a fire's end, the pit air temperature is entirely because the KK walls are hot. Most strikingly with pizza, cooks partly rely on radiant heat from the walls. There's no primacy to understanding the KK air temperature rather than the KK wall temperature.
I can back my car into my driveway using any of three mirrors. We can cook following any of these temperature curves. Hey, I wish a Star Wars hologram could pop up over my smart phone, a 3D version of the weather charts at meteoblue.com, to help me visualize the interior of my KK. Not yet, we're still looking at one number. My point is there's no right number.