If you think about it, it can only be because of development costs related to testing and implementing on two different consoles compared to one. Not the actual computational burden on the console itself.
Sophy 2.0 was already available to a much broader range of cars - literally most of the cars in the game - for a majority of the tracks.
So why would Sophy 3.0 be more of a burden for the PS4 given its for a fixed selection of races with a fixed selection of cars?
It's just Sophy 2.0 with the emojis turned off and some scripting like ensuring that players go from pole to P4 at Tsukuba.
The reason that it's PS5 only can only be that for each update they have to test for compatibility on two different consoles which is just overhead on development, not a technical limitation on the console. And fair enough, the game should've never been released on PS4, it has held the game back regarding several features. Lobby sizes, for one.
It's a significant overhead to put out updates for a PS4 and a PS5 version of a game.