It's not secret, but it's not exactly easy; you can find one part of the medallion just by exploring (iirc its description says what it's for), but not sure if the second half is that straightforward.

And even then, while an item may say "use it here", if it's been dozens of hours since you got it you may just straight up forget about it.

That said, I think that the target audience for the DLC was always people who thoroughly played through the game.

Same with the other games' DLC; for Bloodborne's you need to be yoinked by an invisible enemy while in posession of an item; while it's very possible people were accidentally yoinked on their first playthrough, they will forever know that the item is a trap and will avoid getting yoinked.

(unless the item in question explicitly said "get yoinked to go to DLC", I forgot)