“I have no idea how this “maximizes airflow”—I suspect maximum airflow would come if the holes from one side were drilled straight through.”
I believe the claim apple are making is that increases the surface area, and therefore it acts as a more effective heat sink, not that it maximises airflow.
It might "maximize" (marketing speak for "improve vs. other designs we tried") airflow given a particular requirement for structural strength. This design should do a pretty good job at being strong while reducing the amount of material (and therefore weight), and since it’s just hemispherical holes milled out from each side, the machining is (relatively) straightforward vs. other possible 3-dimensional shapes.
It sure doesn't look like the case is touching any components thus acting as a heat sink.
On the monitor it’s a pure heat sink. On the computer probably not so much.
This makes a lot of sense. High nit monitors, are extremely hot. (they’re basically light bulbs), and the hotter they get the shorter they last. In that case it makes a lot of sense to maximize airflow and heat dissipation, not just one or the other. And from a design standpoint it does so in a very thin space which is important because equivalently bright monitors are about a half a foot thick if not an entire foot thick. I’d imagine the effects are still helpful for a hot computer as well. People are really under appreciative about the design and engineering work that went into building the latest Mac Pro, the monitor (and even the stand), but I realize I’m probably in a very small minority of people.