It's probably not surprising you can't disable the integrated graphics - on many systems, this is the only one connected to the display. Rendering on the discrete graphics would then still require the integrated gpu to blit the data out of memory from where the dgpu wrote the image to somewhere where the display block in the igpu can access it to show on screen. This is also why many less intensive tasks (like rendering webpages or other UI interaction) do so only on the igpu - that blit costs time and power, so if you're not heavily using shaders or are otherwise memory bandwidth bound it may be /slower/ to use the second gpu.
What exactly happens if you select the "dual graphics" option in vulkan?
What exactly happens if you select the "dual graphics" option in vulkan?