Why this is the strongest possible outcome
Termination of a case on the ground that the elements of the offence are absent is a rehabilitating ground. In Russian procedure that is a specific legal status, not a form of words: under Chapter 18 of the Criminal Procedure Code it carries the right to compensation for the harm caused by the prosecution, including property loss, and to restoration of other rights. It is materially different from the more common outcomes, which end a case without clearing the person.
Why the appeal stage made it possible
The appeal court is the last instance in the Russian system with full powers over the facts. It can re-examine evidence, hear witnesses again, admit material that was not before the trial court, and substitute its own decision. The cassation courts above it cannot do any of that. This result was available at appeal and would not have been available two months later.
The fifteen days
The appeal period is fifteen days from the verdict, or from service of the verdict where the defendant is held in custody. Everything described here depended on that deadline being met.