This is not save file modding or hacking. This is using the USB and/or Online backups and restoring them from online. Here's how it's done in Dark Souls:
Player 1 gets all the items needed for most of the trophies and essentially plat's the game first, taking a normal amount of time. They also amass a number of soul items and crafting material.
Player 1 backs up their save online.
Player 1 trades everything to Player 2.
Player 1 leaves the session, closes the game, and restores their save from online (thus restoring all of their items).
Player 2 can then either play the game, or repeat the process above until they have enough materials and/or souls.
You can do this process back and forth as well, to essentially double the amount of items you have infinitely. Say I have a Large Soul of a <whatever they're called>:
Player 1 starts with 5 of these.
Player 1 uploads save, trades to Player 2, and restores save. Player 2 has 5 soul items.
Now, Player 2 uploads their save and trades to Player 1. Player 1 now has 10 soul items. Player 2 restores save.
Player 1 uploads save, trades to Player 2, and restores save. Player 2 now has 15 soul items.
Rinse/repeat a few times and now you have 100's of soul items, letting you get above Level 100 near the very beginning of the game.
If you started with 1 item, it would look like this: 1, 1, 2, 3, 5, 8, 13, 21, ... (Fibbonaci Sequence FTW). You can see how fast it scales!
This technique is called "save scumming" -- utilizing the backup/restore feature of the PS3/PS4 to assist in trophy hunting. Although some may frown upon it, it is widely used in trophy hunting and not prohibited. It's used mostly to trade items, make multiple save points, or follow multiple "decision" based trophies in a single playthrough.
EDIT: Oh, and even if you don't agree with using save scumming to amass large numbers of soul items and crafting materials, trading in Dark Souls is very much normal and expected. A person could still achieve extremely quick plat times if they got the required items from friend. The person playing still has to be good enough at the game to actually get to the end for these quicker times.