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He doesn't explain. You might simply see it as a little quirk. Maybe he feels he has more control on this rather important note on top, to play it tenuto style and give it a beautiful sound. Or he simply wants to make sure he doesn't slide off that black key ;-). It is a soft passage, so it's certainly not about power. A good example for the latter would be the very last note in "L'isle joyeuse" deep down in the bass that needs to be played fff.
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Hi Everyone!
In this case, indeed it is just a "personal preference" sometimes to really confidently play a note, we support the 2nd finger with the 3rd finger. It is important for louder notes and passages for sure, but sometimes we want to have a firm finger tip for entry into the key on delicate passages.
I can understand in this passage, it "feels nice" to play the key that way!
You can absolutely play it just as beautifully with one finger! :)