That advice is only good if you are actually guilty (or if you get arrested).
By going to the police and cooperating, they ended up his side, and worked to figure out who the real person was.
If he has gone all lawyered up and refused to talk to them he would have essentially confirmed everything that they assumed they already knew. They would have arrested him, and only by the actual trial would the truth come out.
But that would not change the past, and he would have still spent time in jail.
If you are innocent cooperate as much as you possibly can - up until you are arrested. Once you are arrested, then, and only then, is it time to get a lawyer. Because if you are arrested then they think you are guilty and cooperating is not going to change their mind.