FWIW, I got the impression that they didn't truncate the table, but just forced an update to it's relfrozenxid to advance the freeze age and allow the database to restart. That means there's a risk that rows that were currently visible would suddenly vanish as the XID advanced and their xmin became "the future". Eventually record of those commits would vanish and presumably the rows would then get removed (though I'm not sure on that without checking the code).
