Without looking at the study details, I would guess that has more to do with predilections of who chooses to go to the parades,rather than being brainwashed by them.
But here is your 4th of July trivia; the following from the article has some fallacies.. Adams wrote that about July 2 being a day of celebration.. The vote on the Declaration of Independence was on July 2nd, with adoption taking place on the 4th. Document was signed in August.
----> On July 3, 1776, he wrote that the Fourth "ought to be solemnized with pomp and parade, with shows, games, sports,
guns, bells, bonfires, and illuminations, from one end of this continent to the other, from this time forward, forevermore."