I think that could apply even if he was chargedIf CB is not charged then there is room for someone else to have done it what ever it is.
I suppose it depends on how you define solved.To be fair there are cases where LE knows what happened but can’t prove it. Often because the body remains hidden.
There have been cases where a defendant is acquitted and police say they are not looking for anyone else, so they must be satisfied that the case has been solved.