Is this laptop using Nvidia Optimus? If so, the displays are only connected to the integrated GPU.
I'm not sure, but the datasheet seems to suggest so, displaying exactly what your conclusion was, that the display is connected only to the SoC. If more proof is needed, where might I find it and what do I do practically? Is this ONBOARD_VGA_IS_PRIMARY or is "PrimaryDisplay" "Display_iGFX?" Or is there something else?