Discrete GPU is actually DRI_Prime=0

I have an older 2011 iMac with a AMD HD6750 discrete GPU and an integrated intel one. For some reason the dGPU is used with DRI_Prime=0 and the intel one is DRI_Prime=1. I verified this with GLXGears output and several tests with Steam games.

How do I force Diablo 3 through Lutris to use this GPU?
Additionally since the dGPU doesn’t support Vulkan (AMDGPU doesn’t support Northern Island gpus), will it revert to using opengl automatically?