EPYC certainly would have made way more sense for the Mac Pro. What with the additional PCIe lanes, PCIe 4.0, lower price and more cores. It also looks as though it can support more RAM.They've made this same rumor so many times and yet it doesn't seem to be happening. Maybe I'm just jaded from one too many cries of "wolf".
I'm half-expecting Apple to jump ship to AMD first before ARM, at least on desktop, partly because AMD's semi-custom solutions is better established vs Intel and could get Apple an exclusive TR or EPYC variant with some missing features like AVX-512 and Registered ECC RAM, and partly because Apple has already been doing business with AMD as far as GPUs go.
As well, AMD is already in a unique position to work in their IF/Infinity Architecture to further blend CPU and GPU as one single, giant APU and being able to share memory to further improve workloads. There's also the fact that Thunderbolt 3 is now royalty-free and included in the USB4 spec, so one less reason to keep Intel.
Apple can still save on costs, while charging the same excessive premiums and banking the difference. AMD would also benefit with Apple funding a bit of the improvements and refinements too to bring even more workstation value to TR/EPYC.