I think the notable difference is that macOS has a standard release cycle of about a year, whereas Arch has a rolling release cycle. This why there's more possibility for breakage, as many of the core libraries or other software are likely having their versions bumped. This is based on my knowledge of how most standard release distros function and which I assume is Apple's update policy for software they ship.

Even assuming this is the case, it still doesn't excuse how they weren't able to uncover this in their testing, since they only have to test against their own set of hardware.