You can tell it's meant to go vertical. Horizontal is just a concession for those that can't really do vertical due to their setup. It's great that they give the choice, for sure.
But my systems haven't actually been "in" my entertainment center since the PS2. They sit to the side, usually on top of their own box to keep them off the carpet. The PS5 will have a leftover piece of 1x6 to stand on to provide stability on the carpet, and its own box next to it (opposite the TV stand) to prevent any falls in that direction (and to put other stuff like the Switch on). Not that I think it's inherently unstable, but I have a cat.