It is a second-class condition, determined as unfulfilled. It is a prophetic figure, the future regarded as past. The siege was shortened by various historical events like the stopping of the strengthening of the walls by Herod Agrippa by orders from the Emperor, the sudden arrival of Titus, the neglect of the Jews to prepare for a long siege. The same deed can be looked at from these different angles. The implication is that it is not possible.
People become excited and are misled and are unable to judge of results.
False Messiahs act the role of the Great Unseen and Unknown. Visible in contrast to the invisibility of the false Messiahs. Like a flash of lightning. Here we have a prophetic panorama like that with foreshortened perspective. Literalism is not appropriate in this apocalyptic eschatology.