If the case had to be retried, Merchan is probably confident of being assigned the case again, but he may not be confident that the trial could be scheduled before the election. He may like the idea of having Trump forced to spend 4 days a week in a Manhattan court room, but may think it is better for Democrats to be able to keep calling Trump a convicted felon.
So the easiest thing for him might be to announce his investigation has shown the post to be a hoax and perhaps inflict some kind of punishment on the poster.