It would be reasonable if anyone could specify the reasons why the PS4 would hold it back. I could say that switching to a vegan diet would hold me back as an athlete, but it's meaningless without citing the reasons why. It's a pretty empty statement to make, because a lot of the reasons that have been expressed so far have been easily debunked. After all, no one claims that consoles held Cyberpunk back on the PC. That was just sloppy development.
As it's a fairly transitional game in terms of mechanics, the limitations are asset creation, and engine optimization. The former is largely budget-limited. The latter is impossible to quantify.
The obvious constraint is that Guerilla needs to make things fit inside the PS4 instead of the drastically more powerful PS5. They can't add as many things to the game compared to if they were to make it PS5 exclusive. Things such as assets, code, textures, animations, lighting info, etc. If you like using analogies, I might be able to give you one you'd understand.
Imagine going to the grocery store with a big sturdy bag VS 1 tiny torn up bag with holes in it. You'll be able to fuel your body a lot more if you bring a big sturdy bag to carry what you need, because it's a lot bigger and a lot more reliable.
Now for the less obvious reasons of why the PS4 holds back this game. The SSD of the PS5 doesn't just make things load faster. I mean it does, but not just the typical normal loading screens that we as players get to see. It actually changes the way the developers can develop the game.
Playstation gave the example that on the PS4 they couldn't make Spider-Man zip through New York super fast. Because in order for you to be able to zip through with amazing speed, they would need to make assets load in fast enough so you don't see super bad pop-in.
Other examples would be long elevator rides, long tunnels to walk through, squeezing the character through cracks in rocky areas, slow camera movement, uncontrollable cut-scenes, etc etc. All these things are used to hide loading times.
Last but not least, every developer would have brilliant ideas they want to try out. However they often only come to know it's not possible midway, because they have to deal with the PS4 also. They either try their best and eventually succeed on implementing the ideas, but having to take away loads of time and resources from other stuff to compromise. Or they would have to scrap it entirely and still would've wasted time and resources regardless.
In short, if the game were to be a PS5 exclusive, the devs would not only be able to make it more graphically beautiful but also design better worlds, add more assets and NPC's to make it more lively, add more code for better AI, add more animations for more fluidity, add entirely new additional gameplay features, AND waste no time on looking for workarounds for the PS4. These are just the things that came off the top of my head, in reality, there's tons more things the devs would need to deal with.