On page 181 of the core rules, the Long Night is described as an age of darkness, isolation, and decay. Trade declined, whole cultures vanished in the sands of time, no progress was made, and populations declined. Then the Zenith arrived. But the Portal Wars that immediately preceded, and precipitated, the Long Night, ended just over a hundred years ago (page 180), and the Zenith arrived 66 years ago (page 248, assuming the current year is 61 CC), so the Long Night was maybe 50 years long. That does not seem nearly enough time for people to forget the advanced technology, for cities and outposts to fall, and civilisation to recede!
And, at the same time, the Nazareem's Sacrifice cult, that was wiped out over a hundred years ago, is still around as sleeper cells in ways that the First Horizon feel confident of being able to contact. Are these multi-generational sleeper cells?
Finally, what exactly did the less than 100,000 people on the Zenith, suffering memory loss and madness from low-tech cryo-sleep, do to kickstart trade and civilisation in the Third Horizon?