They never said that the 9%-12% represent efficiency. They said in the video that at idle power less than 20% or the rated wattage, the efficiency is allowed to be less than 80%. The 9%-12% are a percentage of power draw with respect to rated wattage. They even go on to say that at a small load like this, the efficiency could be as low as 50%-60%.
I'm no expert electrical engineer but I know enough to read the simplified schematic on wikipedia and see that the 400V after the PFC part is still AC (No transformer) and that there is a big transformer after that part with multiple separate secondary windings for 12V, 5V and 3.3V.