The police knowingly pursue a man they know is not Montag. The media knowingly broadcasts the pursuit and capture of a man they know is not Montag. Though the actual capture of Montag is important, what is of greater necessity is presenting the state in action. The citizens must know that the government is watching and will pursue those who violate the repressive laws that are in place. The pursuit and capture of Montag's substitute is an example of the consequences. It doesn't really matter if it is Montag or not. The example is all.
This pursuit symbolizes the manipulation that the state uses on the citizens. They portray what they want them to think, rather than what has occurred. The are creating the reality that they want the people to live in and respond to. As TV has become a virtual reality, so the news has become a virtual reality.
This is epitomizes the maxim that one does not respond only to his own reality, but even more so to the reality as seen by others.