Ultimately, the decision to prosecute or not for Disorderly Conduct will be made by the Public Prosecutor's Office (as it said in the story).
Using disorderly conduct statutes to control public behavior (such as rowdiness and public swearing) are effective because the goal of the police officer is to stop the offensive conduct, not because they disagree with the speech, but because they want to prevent the inflammatory speech from inciting violent confrontation.
Once ordered to desist, a person who fails to desist can be arrested and removed from the venue to prevent confrontation. This solves the immediate problem even if the offender isn't prosecuted or simply receives a fine (disorderly conduct is a misdemeanor).
However, with a printed message, such as a billboard or a bumper sticker, which is more permanent, any attempt to prosecute will just wind up in escalating court proceedings. Something no prosecutor's office has the time or staffing to deal with.
This guy will just keep driving around with the sticker until he's eventually assaulted (or not) and then he will appeal to police for recompense. A judge will then decide if any assault was sufficiently provoked.