Hello again, I've attempted an experiment with the vendor firmware to get to the bottom of this.
With the vendor firmware, Primary Display can be set to: Auto, IGFX (possibly PEG and PCI. I haven't tried these, it also may be one or the other that appear) and SG. There is an option to "Select PCIE card," which describes the behaviour of the DGPU_POWER_EN pin. (Auto: Skip GPIO-based power enable and Elk Creek 4 and PEG Eval, for DGPU_POWER_EN is ActiveLow or ActiveHigh).
In any configuration of "Select PCIE card" with Primary Display set to SG, the dGPU appears. However, in any configuration of "Select PCIE card" with Primary Display set to IGFX, the dGPU does not appear. The default configuration in the vendor firmware is Primary Display: SG; Select PCIE card: Auto.