I do not think they are talking about source code, rather about some compiled code cache. I am not completely certain that the author of the article knows what they are talking about (but I am quite confused myself and I will appreciate an explanation).
Also, I thought a lot of the Darwin MacOS kernel had already publicly available source code.
Some of the kernel is released months/years later as open source. You are correct about the releases being macOS only, iOS xnu has never been open sourced but it is, for the most part, identical. Apple has also been moving code out of the open source kernel releases and into private, closed source kexts. Kernel extensions like Sandbox have never been released.
I forgot to mention launchd. It was open source then was closed and split into launchd/libxpc. It has always been a critical security component of macOS/iOS. Many CVEs have been written about it even after it was closed via binary reversing and fuzzing. Having the source again would be nice.
Also, I thought a lot of the Darwin MacOS kernel had already publicly available source code.