Apparently Apple had macos working for years on x86 before they switched their computers to intel CPUs. The justification at the time was exactly this - by running their software on multiple hardware platforms, they found more bugs and wrote better code. And obviously it made the later hardware transition to intel dramatically easier.

I would be surprised if Apple didn’t have internal prototypes of macos running on their own Arm chips for the last several years. Most of the macos / iOS code is shared between platforms, so it’s already well optimized for arm.