According to docs, enabling bitcode: "Includes bitcode which allows the App Store to compile your app optimized for the target devices and operating system versions, and may recompile it later to take advantage of specific hardware, software, or compiler changes."

It seems quite likely they have (and probably used) the capability to recompile any app on their platform to benchmark real workloads against prototype silicon changes.

It's enough that enough of them are.

Plus, even if most aren't, the "short tail" people use these days probably are.

macOS promotes the App Store as the source of software (even if it's not the sole), and has walls like notarization requirements and the Gatekeeper to prevent weeds from intruding.

With the App Store Apple knew that there's a pool of N apps that follows its guidelines, has passed internal checks for API use, and can be converted quite easily to a different architecture, that it could count on.

Their control over the platform allowed them to enforce Metal and deprecate OpenGL pronto, to add a new combined iOS/macOS UI libs, to introduce Marzipan.

They have also added stuff like Universal Binary support, and most importantly Bitcode, which abstracts away parts of the underlying architecture.

All of those where steps towards the ARM/M1 (and future developments), and all were enabled via Apple's control of the hardward, software, and - sure, partial - control of third party apps.