The influence of the Polish music (not only folk music) is actually well documented, it is definitely not only a cliché resulting from his political involvement in the Polish independence movement.
It is known that, at a young age, Chopin has been fascinated by Polish folk music. Please note that his teacher (Josef Elsner) was author of two major studies concerning "rhythm" of Polish language and folk melodics (although Chopin criticized his approach). Also Chopin's letters indicate that he used to participate in folk events in order to become familiar with this music (and that he was taking notes).
Chopin's music include traditional Polish motives from kujawiak, mazurek, krakowiak and piesni ("songs") - i.e. folk music, as well as gentry music (for instance polonez).
Of course the use of Polish folk motives is not the only reason why his music was so distinctive.