Tradition from Chopin
Fully preoccupied with Chopin's method, particularly Chopin's phrasing, he liked to say he wore out his fingers on the posthumous Étude in A flat of the Polish master, whom he ranked with Bach and Liszt. [Long p. 19]
It is one of the most happy and lively memories of my artistic life to have heard him play a number of Chopin's pieces, Chopin being a composer of his particular predilection and one whose every secret he marvellously divined. Until he informed me of this fact one day I was utterly unaware that in his youth he had worked long at the piano with a pupil of Chopin's, and he explained to me how considerable a part this instruction had played in his musical formation, not only as a pianist, but also as creator. [Nichols p. 96]
The above mentioned pupil of Chopin refers to Madame Mauté de Fleurville, who was the piano teacher of Debussy in his childhood. There are no indications in the sources that she really was a pupil of Chopin's, so this is doubtful.
Debussy's style of playing seems however to have several similar characteristics to Chopin's, like the carezzando-touch, flexibility, a soft touch, delicate nuances, refined use of the pedals, and a controlled rubato. [see Eigeldinger for the style of Chopin].