That PCI slot has low power output, not suitable for what people would plug into it. Easier to cover it than run into support problems. It won't run a GPU directly. It will run an Oculink card which will allow use of an egpu.
I can't imagine why you'd want to buy a beefy SOC with unified memory, only to have it host a discrete GPU through it's narrow PCIe x4 interface. You'd be better off with a traditional CPU that supports a proper x16 slot.