Well, think of it like this.
The Apple (Mac) Silicon development didn't happen over one year, or even two. It was a multi-year effort following the realisation that there was more to be gained from sharing one common architecture.
What's key is that although Apple never confirmed they working on their own Mac processor, the writing was on the wall for years - we just never put two and two together.
It was an incremental transition. The introduction of such features as the Mac App Store; the under-the-hood and cosmetic changes that came with Mavericks; the renaming of apps and iCloud integration; and much, much more. Many of us, myself included, questioned this "iOS-ification" of the Mac, but it was in fact all part of a larger plan to share experiences.
I would say the biggest giveaway for AS development was actually in Catalina, the previous macOS. This was quite simply the buggiest macOS I've ever used, and many thought the same. But why was this the case? Why such a dramatic change in reliability?
There's no question in my mind that Catalina was a prototype for Macs running on ARM - the first 'fully' compatible version of macOS for AS, and they would have been using it internally as a testbed. Even the introduction of the Catalyst initiative to, as Apple put it, "encourage developers to bring their iOS apps to the Mac" (Hmmm can't think why!) and the changes in U.I made to much of System Preferences that mimics iOS Kit.
So Apple has had plenty of time to get this right. They've been very fortunate to have an OS that, upon it's very first release, was already suitable for running on multiple architectures (Brilliant foresight, as per the PPC > Intel transition), and they have the resources to produce as many hardware prototypes as they've wanted over the years.
The future is very bright indeed!
