Guilliman has the authority to name someone Fabricator-General?The Errant armor is basically just an upgraded version of the Mark VII. Not really an innovation. Besides, innovation is still considered tech-heresy by the Mechanicus even if they grit their teeth and accept it at times. It's why Cawl is such a controversial figure in the fluff: he actually goes out and invents things, which most orthodox tech-priests think is an affront to the Machine God. Guilliman refuses to name him Fabricator-General specifically because he knows doing so will cause a civil war in the AdMech.
I wish I hadn't heard anything about the fluff post Dawn of War's release.