I see what you're saying. If they convicted Mangione, and the real killer confessed, they'd likely just ignore it and double down on his guilt out of vanity. That's what usually happens in reality.

But it would not be a good idea for the real killer to confess. Yes, the video shows that one physical human wielded the gun, but as I am pointing out, that doesn't matter. That's the fucked-up part here: The legal system would allow two people to be convicted for it, unless somebody took deliberate action to overturn the first conviction.

It might go like this: The NY prosecutors get a conviction, and send Mangione to prison. Then, the real killer confesses. Being the real killer, the confession is very, very credible. The prosecutors can't brazen it out, and decide to charge and try him. If he pleaded guilty, wham, bam, done. The matter would go right to sentencing. If he pleaded not guilty, the judge might rule that the conviction of Mangione for the same for crime is not admissible evidence, as it might bias the jury. Even if the previous conviction was brought up at trial, they have a very credible confession; it's clear the real killer did it. And, even without a confession, the jury in the second trial is supposed to consider only whether the defendant is guilty of the crime charged. And if the evidence is strong enough, they probably would. The conviction of Mangione for the same crime would be a matter for another court.

Since the Supreme Court has repeatedly ruled that innocence is not grounds to have one's case heard before an appeals court, this scenario could easily result in two men in prison for the same crime. The outcome of both going free is highly unlikely.

Yes, in reality, the NY prosecutors probably would at least have enough integrity to move to overturn Mangione's conviction if they had a credible confession from another man. But they don't have to.