It’s "Mac OS", not "macOS", or even "MacOS" (which is the title on the actual page).
That said, this is some serious wizardry. I wish I had one of these to play with.
The HN story autoformatter changes titlecase on well known terms, which I'm betting is what happened here.
This is correct, and interestingly enough in one of the screenshots, the OpenFirmware prompt says "To continue booting the MacOS" too.
https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh...