IMHO, the big thing was the mismatch between what Apple wanted in a CPU and what Intel was prepared to offer the marketplace, and AMD wasn't different enough to matter.

Intel/AMD chips are designed with one thermal target for acceptable computing and a second, much higher target if you want to compute at the highest throughput continuously.

Apple did not provide the highest thermal capaicity and suffered when comparing similar cpu against another OEM. With Apple silicon, the cpu is designed around the thermal solution Apple is willing to provide. A lower power target leads to a lower clockspeed target leads to different design tradeoffs than Intel/AMD where flagship designs must clock to the moon. You can see similar benefits for the lower targets in AMD's ZenC cores.

But ZenC wasn't available, and Apple probably wouldn't want to be running laptops with only ZenC when you could get a regular Zen laptop from someone else. Apple benefits from avoiding apples to apples comparisons.

Likely Apple won't lean too heavily on Intel fab to start with. Let them do processors for value products and see where it goes, but always plan for fab agility. At least until Intel fab becomes a reliable partner.