That's not really 100% true, take Cyberpunk for example, they started it on PS4/Xone but now it's getting delayed because they ended up making it ideally for the next geneartion and now have to go back and find a way to make it work on the previous one.

It's likely that Guerilla Games will make Horizon:FW for the PS5, then downgrade it to make it workable on PS4.

Since this generation is basically just a major step up in computing and graphics power, it's not that difficult for them to make a game that is just designed to run on the base architecture of all the current gen systems and PC, and then tailor the experience to each system's level of power.

Basically it's just much more like PC game development, the added graphics and resolution options in PS5 games is already an apparent example of how easily they can tune the graphics to exactly where they want them for the hardware.