The last bass (Ab) before the nine final chords: there is a crescendo marked on that note (it begins and ends with this note).
Try to play it as is written......it's an example of the idea, of the poetic intent that transcends the reality (of the instrument's possibility).
The poetry (of music also) is a re-invention of the reality, and some details (as this, which forget the physical reality) exist to remember us this.
But one can sing this wonderful and not real crescendo, one can (must) think it when plays that Ab.
These "Quasi Variazioni" were born before the other movements of the "Concert sans orchestre" , and have an autonomous life.
IMHO (but I've read other similar opinions) this is the only case of a Schumann (or Wieck/Brahms) late edition which is better than early edition.
As Schumann, also Vladimir Horowitz began from these "Quasi Variazioni". Subsequently he played and recorded the entire Sonata, but he declared that these variations were the key to comprehend the beauty of the Sonata.
I do not know a better recording of this wonderful piece than Horowitz's. Also the other (in reality my first and favourite Vladimir, and my favourite pianist among all) Vladimir, Sofronitzki, played these Varations (not the Sonata), and, obviously, it's also here a good trip. But here the most fascinating, coloured, fantastic and schumannian Vladimir is Horowitz.
Schumann - Op.14, Piano Sonata in F minor, III. Quasi variazioni. Andantino de Clara Wieck