Apple open-sourced the kernel of iOS and macOS for ARM processors
Apple has always shared the kernel of macOS after each major release. This kernel also runs on iOS devices as both macOS and iOS are built on the same foundation. This year, Apple also shared the most recent version of the kernel on GitHub. And you can also find ARM versions of the kernel for the first time.
So the fact that you can now download ARM-optimized source code of Apple’s kernel doesn’t mean much. Maybe Apple wants to share the kernel of the iPhone to get feedback from the open-source community. Maybe it means that Apple is working on a version of macOS that runs on ARM chips. Maybe it was an accident. Maybe Apple just wanted to see the reaction on Twitter.