If you've forgotten something then you don't have that knowledge anymore. So remembering things for longer means you are effectively gaining knowledge.
It's like someone giving you $10 and then trying to argue that they haven't actually given you $10 because just a few minutes ago you had a $10 bill in your pocket but you spent it on something else.
It's like someone giving you $10 and then trying to argue that they haven't actually given you $10 because just a few minutes ago you had a $10 bill in your pocket but you spent it on something else.