it's not. it is for the version they made on PS5, but not in order to do the stuff they do in the game. you can always simplify assets in order to fit into ram or load relatively fast from a slower drive.
as an example (an extreme one but still), Super Mario 3D World on Wii U was designed in a way that almost all the important assets of the game would fit into the Wii U's RAM pool at once. this meant that the game, once you are in the world map, has almost zero loading times. in fact, they needed to artificially slow down level transitions in order to give players time to join/leave the party and choose their character. and even with that artificial slowing down, you are in a level within less than 4 seconds, loading FROM A BLURAY DISC!
Mario Galaxy on Wii loads also in about 3 seconds into a level, sometimes faster. if you want fast loading you can have it, if you actually tailor your game towards it. and it doesn't matter how slow your storage medium is. the Wii can load 8MB as second...
fast transitions are not impossible on a slow harddrive, the issue is that they would need to optimize the assets in order to work on the slower drive.
all you have to do is size assets accordingly, maybe try and reuse as much as possible between levels and if your CPU sucks (which is the case for the PS4) then having as much of them uncompressed as possible is key
edit: an example of a modern game,
here's a clip of someone fast traveling in Doom Eternal level in about 2 seconds on a PS4 (On One X this is even faster):