You are granting too much credit to the fiction that the authors of the game wrote into their own setting, which was compounded by GT and T4 "third party"authors completely missing the "read between the lines unreliable narration of Imperial propaganda" that is the setting as told from the Imperial perspective. Put bluntly the Imperium lies, and if you step back and look at what really happens you can spot the lies.
Since Mongoose now decides Third Imperium canon it has to be accepted, except the statement is self contradictory, and likely based on taking T4 and GT at face value, as I will show
Only the Emperor can create nobles, although in the past this authority was granted in a limited way to archdukes, but their powers were removed centuries ago. The Moot has nothing to do with making nobles but it does confirm heredity inheritance, and their recognition of the Emperor is a forgone formality as the Emperor controls the military and the taxation.
The MT IE has a referee secrets section that is often overlooked, here is what it says about the Moot:
"Moot, The:
The Moot is not as important as the SMART library data entry implies. While the Moot does confirm emperors and high noble appointments, it did not act to confirm those emperors who ascended by the right of assassination."
The Emperor is supreme, the Moot has no authority over them.
They can never use their much vaunted threat to dissolve the Imperium*, and yet the Emperor can strip every noble of their title with a wave of their hand. So who has the authority?
The Imperial military swears allegiance to the Emperor, not the Moot, so who has authority?
*they can't use it because to do so would trigger another civil war, with fleets loyal to the Emperor, fleets loyal to the pretenders, and scallywags willing to sit it out and wait for the dust to settle.
