Plus Knights are grouped into Forgeworld-based Households and are part of the Adeptus Mechanicus - not the Diviso Militarus which commands the IG at all.
I certainly want to see them, but them being in the IG codex would be similar to titans appearing in the Space Marine codex - they just aren't part of the same organization.
It will be interesting to see how this pans out and how GW shoehorns them into whatever book they go into. Who knows - maybe they will get special rules of their own to be attached to any Imperial army. I could get behind that.