My guess is that, if AMD is involved, Apple would license AMD64 & Zen micro-architecture and implement it themselves. AMD has licensed AMD64/Zen before (THATIC). So it's not a new idea for AMD. Also, for Apple, it won't make any difference whether they use ARM ISA or AMD64 ISA as long as they design and manufacture their own chips. I think software compatibility is much more important to them then ISA religion. It may even make sense to have both ARM and AMD64 cores in a single chip and run both iOS and macOS using their native ISA.
When they transition from PowerPC to Intel, Intel had performance edge that smoothed out the transition period. I don't see Apple chip providing such performance edge over Intel in the near future. Even the best ARM server chips are not as fast as Intel core per core and single core performance still matters for desktop use. If Apple is ambitious, maybe they will finish what AMD started (one micro-architecture that can handle both ARM ISA and AMD64 ISA).