Yes holm you're correct. You can use a USB for save-states for PS4 games on PS5. If it was just a PS5 only game, you can only use a cloud save. Funny enough I think PS4 user interface (XMB) has an advantage of a shortcut button to go the save/load USB settings.

Only issue with USB save-states, is that the game save file limit. Not all areas that you want to practice would exceed the game save file limit of 10. This means you can either spend extra time and have multiple save files on computer then plug your USB into the PC, get the other practice file, put it in your USB, take it out, put into PS5 and play; or you can have multiple USB's.