Merry Christmas Raymond....Often legends, like prophesies, are reverse engineered, that is, something exists or happened, and a backstory is created to explain it. In this case, after the events of the First Jewish-Roman War occurred, it seemed appropriate for Jesus to have predicted it. Mark 13 was, I suspect, recast from an apocalyptic standard end-of-days piece into a specific prophecy of the war with the addition of just a couple paragraphs spliced in awkwardly. It has to be significant that no early writers mentioned this rather significant 'fulfilled' prophecy prior to Tertullian. Even Justin Martyr when supposedly debating the Jew Trypho argues for the superiority of Christ by citing his remarkable prophetic insight to predict the suffering and hardship of Christians. He somehow forgets to mention, 'Oh Yeah, he also predicted Roman armies were going to destroy your Temple and Jerusalem!'
There is an interesting reference in Josephus to a mass evacuation after the 66CE routing of Celsius' garrison at Beth Horon. Hyper-nationalism among the Jewish victors compelled citizens that had supported the Romans to flee in large numbers. "Distinguished Jews abandoned the city like swimmers from a sinking ship" (War 2.20.1 556).
It is entirely reasonable to believe any Christians living in Jerusalem left at that point if they felt threatened in that environment.
A distorted memory of that might have played into the enhancements of Mark 13 and the legends that Christians fled by divine revelation.