Try your comparison through a full series of temperature. When I set my controller to 225F, I find the 3rd party thermometer reads about 5 degrees low (220F). If I bump the temperature up to 325F, the cook temperature will be about 300F. If I want to cook at 400F, I have to set the controller to 450F.
If there was always a fixed offset in the temperature then calibrating the thermometer would be easy, but there seems to be calibration slope that is not so easy to calibrate.
When you calibrate the Traeger internal temperature probe, you insert it into an ice/water bath at 0C/32F. Unfortunately, that is well below the range of temperatures useful to us during cooking. Thus, I never considered that calibration to be useful.
The long probes used in some older models might be waterproof such that calibration might be useful. The newer probes are short, not easily removed and are not waterproof, so calibration is not possible. The best you can do is learn how your specific grill operates over the useful range of cook temperatures. Once you do that, you simply figure out what you want your cook temperature to be and adjust your controller temperature appropriately. Even if your controller has to be set 50F high, that is only an issue if you are trying to achieve a cook temp above 450F.