I'm pretty sure this is about game-specific uprezzes \ framerate upgrades \ texture upgrades \ etc.
Trust me, most games definitely have room for low hanging fruits upgrades - graphic features that have been cut for framerate, etc that can be swapped back on on more powerful platforms.
This is what's happening.
I think this just about games working on PS5 from the get go. Devs can make improvments if they want but it will not be mandatory.