Dereck I agree with you but I do not think that is what is happening here.
Check out my response.
If you run 8 AWG for the 'hots and neutral' you will have to run a 8 AWG EGC to comply with 250.122(B).
If the neutral alone was increased as is the case with much of the 'super neutral' MC we run the EGC can remain the size of the ungrounded conductor.
Specialty RMC
can be used as the EGC per the NEC.
That aside I do agree with mxslick that for underground or outside runs it is better to run a 'wire' EGC.
I also would be running PVC in the ground but that is another personal choice.
Bob