It's somewhat not surprising that Mr. Barboza elected not to appeal his criminal conviction and sentence, as he (1) pled guilty, and (2) was sentenced to probation (with his six-year prison sentence suspended). Not a bad result at all for someone who pled guilty to robbery as well as a felon-in-possession enhancement.
But Mr. Barboza was a minor whose case was direct charged in adult court, and Prop. 57 was about to be voted on by the electorate later that year. If Mr. Barboza's attorney had filed an appeal, he'd have been entitled to retroactive application of the statute, and might have gotten some relief.
But no appeal was filed. Which meant that Mr. Barboza's conviction was final once Prop. 57 was passed.
Which means he can't request a remedy under the statute.