It doesn't mean you actually did, but it does mean the system says you did.
Which is the point of my comment. It doesn't mean it actually makes sense in real life, but it's why the system does what it does.
And the reason why the system will prosecute kids like adults sometimes - the nominal reason is because they don't think it's worth giving them a pass either due to the severe nature of the crime, or because of the lack of contrition of the accused.
If they're actually innocent, then per the system they should get zero penalty eventually regardless of how they are prosecuted.
We know that being held in an adult jail while awaiting trial is a pretty severe penalty in fact of course, which is why it eventually got thrown out that he got treated that way. Plenty of adults get stuck in jail for years while awaiting trial, then get released and theoretically suffered no penalty either. But we also know that is bullshit. No clear better alternatives (except bail) have shown themselves however.
If the kid had been caught on tape murdering a bunch of other kids and still claimed he was innocent, then no one would be objecting that he be put in an adult jail while awaiting trial though. Since putting someone that violent in a juvenile facility is making it as dangerous as an adult one.
Judges have wide discretion to make these calls, and this judge clearly screwed up.
But that's the how and the why.