Hello,
I play this piece and am able to get a sparkling pp.
How I do this is very easy and simple to do but difficult to explain but here goes lol...
You need to keep the wrist low and the fingers must touch the keys before playing the note, the muscles must feel the resistance of the keys and aim for the release of the hopper by actually feeling and listening for when the hopper releases with each note.
Play the key very slowly down and it will stop at a point, that is where the hopper releases.
if you can feel the weight of the key then you will be able to resist that just enough using your own weight to give a very true pp.
Not only that, the pp will not be sparkling unless your fingertips are firm and sharp like the blade of ice skates, you must have a pulling action with the last joint of the finger, even though the finger is going straight down because of the upward feeling at the knuckle.
You must use rotational pressures for each note if you want this to be effortless.
generally rotate at the direction you are travelling except for the starting note (which is a rotation opposite to the direction you are travelling) and except when you pass under the thumb or over a finger (which would be in the direction of the side of the hand, the finger side or the hand side). That is a complex way of finding the right direction to rotate, the easy way is to simply experiment in both ways for each note, one way will be comfortable and the other way will be uncomfortable. Like you can when standing lean on one leg or the other without an actual movement, this is what you must do with a piece like this with such speed, it is the secret to a crystal clear tone.
So in a word, no weight in the hand (by being close to keys, wrist low, touch notes before playing them) and feeling the upward pressure at the wrist and knuckles of the weight that must be released in order to just break through the actual weight of the key in order to produce the quietest pianissimo, aiming this weight for the release of the hopper which you can feel with your muscles and you can hear which is when the sound starts.
Sorry I cannot be more simple than that in explaining but it is something that once you understand is very easy, great pianists do this naturally with their great piano instinct, others have analyzed this and it seems complex but is really as simple as a heart beating. (which anatomy books make seem complex)
FCP