If the GRT Mini has a magnetometer, you don't need the separate compass. If it doesn't, you do :-).
After looking it up, I am surprised to have to agree with you on this one Mark. 91.205(b)(3) only specifies a "Magnetic Direction Indicator". I'll buy that!

The Minis do have a magnetometer, but the compass was already there. No harm in a small redundancy, eh?! 😁