We're talking about a dedicated gaming system here, iGPUs are pretty much irrelevant in that context. Things would be different in the context of a browser or word processor.
Besides, counting "1 Intel CPU sale = 1 Intel GPU" is misguided since that would count everyone using an Intel CPU with a dGPU as an Intel GPU customer, even if the iGPU sits idle 100% of the time.
Looking at a more representative demographic, the Steam users[0], Intel has a 9% market share, vs AMD's 15% and Nvidia's 85%. I don't know whether this also overcounts hybrid-GPU users (Optimus/DSG) into the iGPU bucket as well.
Parent comment is thus irrelevant at best, misguided at worst. Not supporting Intel GPUs is throwing away the dominant leader in the GPU market.