Does anyone actually care? This was embedded software designed to run on exactly one system, not some libc designed to run on 20 architectures ranging from 8-bit microcontrollers to VLIW supercomputers.
What I find interesting is looking for creative ways to prioritize the game experience even when the 'official spec' didn't support it. They could have given up by accepting that the spec represented the limit of what could be done, but instead pushed to find a better way.
I hadn't put thought into how important that scroll effect is to the game, but if there was a clean wipe between scenes it would have been tremendously distracting. This technique really is essential to the feeling of immersion.
I don't agree with the suggestion that there is no difference (other than word semantics) between relying on empirical observations and relying on a privately communicated piece of spec.
The one playing semantics is you, since your apparent concern is the definition of the term "undefined behavior" and whether something falls under that definition.
My point is about how well an engineering decision is justified, not what term applies to it according to some document.