Also, there's a lots of experimentation in Chopin that surely influenced Debussy. In the Polonaise-Fantasy in A-flat there's a prominent octatonic passage in the melody, plus loads of sequences through his output that make use of octatonic patterns (like the twice-repeated octatonic ascending arpeggios right before the code of the F-minor Ballade). There's the black key etude that made a game out of playing a pentatonic melody using only the black keys (at least at the beginning of the piece).
There is also Chopin's prominent use of the arpeggiation technique where you move up the keyboard by bouncing your pinky then thumb on the same note (or bouncing your thumb then pinky when going down).
Plus all kinds of little games like taking a slow moving bassline from one section and speeding it up 4x in the melody over a seemingly improvised chord progression to transition back to the theme.
Just with those examples you've got all the ingredients necessary for something like "...Brouillards" from Book II of Debussy's Preludes.
I don't think the significant achievement of Debussy is some mythical genius who came out of nowhere. Rather, I think it's someone who was heavily influenced by other composers like Chopin-- so much so that you can feel the influences as you play his music. Yet the sound of the music is mysterious because he's combining those influences in novel ways that don't follow the harmonic or even formal paths of those influences at all.
So "black keys only" becomes "black keys in the right hand and white keys in the left." Harmonies built from octatonic scales don't function to lead to a new key but instead just sit where they there. And Chopin's bouncy arpeggios just drop off and lead to yet another mysterious section of music.
Those are all big chances to take because you can immediately hear the influences, but if you don't continue to investigate the music everything just sounds weird and "wrong." I imagine that explains the kind criticism Debussy was getting from his teachers-- not necessarily, "I don't understand at all what you're doing," but, "I hear what you're doing but I don't think it leads to anything other than novelty."
I think that's an understandable criticism for the time. Imagine hearing for the first time a piece where one type of music interrupts the previous music, then it just goes away. Is that just a parlor trick to laugh at, or is it a burgeoning new way of thinking about musical form? We know the answer because we can read music history books, but that's cheating. :)