Correct, MBP's can run stable diffusion and other ML workloads on non-nvidia hardware. I clearly see this becoming a trend. GPT-J, Neo and NeoX run really well on Colab TPU's, again these are not made by Nvidia.
Training is dominated by Nvidia, I will not question that as most papers I have seen say something similar. I will say that I do not believe training will always be dominated by Nvidia's datacenter options. Two things that will hasten the withdraw from Nvidia; Cuda and hardware advances around the motherboard (ASICs, RAM proximity, PCIe lanes, data transfer planes, etc).
Think about this... what if a company released an ML training/Inference ASIC that used regular DDR4/NVMe, performed like 4 x A100's and cost $8000? Would you be interested? I would! I don't think this is too far off, there has to be someone working on this outside of Google, Apple and Meta.
StableDiffusion alone was trained on 256 x Nvidia A100 GPUs.