However, the judges could intimidate the witnesses who saw the new moon into not testifying. When they did so, the outgoing month would be extended to the length of thirty days. They were not allowed to shorten the month by telling the witnesses to testify that they did see the moon when they actually did not. To do so would be telling them to say an outright lie.
You could also argue the opposite entirely. To tell the witnesses to suppress the testimony would be an outright lie. If the moon really was out, many other people saw it too. If the court did not sanctify the New Month, it would look like a falsehood. But to tell the witnesses to testify that they saw the moon when they, in fact, did not - that was OK. Because in this case, the people would think that the witnesses were lucky and sighted the moon before others. Thus it comes out that they could shorten the month.
Art: Man and Woman Contemplating the Moon by Caspar David Friedrich