For a Vulkan application, DRI_PRIME=1 only moves the selected device (dGPU) up the list of devices which the application can choose from. This is different from OpenGL where DRI_PRIME in fact replaces the device shown to the application.
(see: Environment Variables — The Mesa 3D Graphics Library latest documentation)
Usually a Vulkan application (as is dxvk) screens the list of devices for certain capabilities and then chooses the one it perceives as most fitting. I don’t know about what criteria dxvk uses for its choice, but I suppose it (correctly) considers the dGPU better for the intended purpose. A native Vulkan app might even expose the device list via their configuration options.
btw the links above also mentions this:
For Vulkan it’s possible to append !, in which case only the selected GPU will be exposed to the application (eg: DRI_PRIME=1!).