I don't think that's an apt comparison, given that Microsoft and Apple were more direct competitors than Intel and Nvidia; the latter have a more symbiotic relationship. I think the rationale is closer to the competitor of my competitor is my friend -- they face two threats by AMD growing larger in the CPU market:

- a bigger R&D budget for their main competitor in the GPU market

- since Nvidia doesn't have their own CPUs, they risk becoming more dependent on their main competitor for total system performance.