An IG investigation would only cover DOJ involvement anyway. Once a link is established to players outside the DOJ, all bets are off.
Much depends on Huber’s special charter. We don’t know what (all) that entails. It could be narrow, just bird-dog Horowitz’s conclusions, or it could be very broad and in the nature of oversight of many investigations including shenanigans by State, DOJ, FBI, CIA, and WH itself.
At some point all of this will come to a head, as the calls for 2nd SC are persistent, and will grow louder as more and more evidence is disclosed and as the public realizes the limitations of OIG.