it doesn't matter what the game pushes.
what is happening is that the game has aspects in its graphics that in some rare cases overwhelm the GPU of the PS5.
it doesn't matter if it pushes any remarkable graphics features or not.
what could happen here is that some of the effects used, especially in the character select, are simply too demanding for the PS5.
why that is is not really important as both versions most likely use the exact same settings and everything displayed on screen is identical on both GPUs
whatever it is runs better on Series X
now maybe it is a rogue graphics setting like that weird auto optimisation setting used in the Xbox versions of Elden Ring (which was thankfully patched out recently) that tanked performance like crazy...
so that would be something you could point to to say you can't use the game as an example of which is the more powerful console.
if they both run the exact same settings
tho, you can totally use it as an example, because like I said, it doesn't matter if a game looks demanding, it can be demanding for weird reason that aren't apparent at first.
could be there's something on that VS screen that leads to a shitload of overdraw (objects hidden behind the menu background, maybe placed there to compile shaders),
or could be that one or more effects used (DoF, transparencies etc.) are just a bit much while looking not really impressive