he custom originated in 19th century, long before the independence that Poland regained in 1918.
According to an article in ethnology that I read years ago, making the rogal a symbol of Independence Day was a conscious strategy to de-emphasize Piłdudski*, who was not popular in the region due to his lukewarm attitude toward the Wielkopolska Uprising without which Poznań would have remained Posen.
*nov 11 is the day he formed a government for the newly independent country