Exactly, the long night was not a time of nuking back to the stone age or even tech regression. Don't believe everything 3rd Imperium historians tell you
It was a time when the Imperium ceased to be, world became more insular and only conducted trade in local clusters, and technological development stalled.
IMHO the reason for the latter is that actually going up a TL is hard, new science has to be discovered, new engineering principles based on that new scientific paradigm have to be developed. It is usually hundreds if not thousands of years between TL improvements
Earth achieved TL9, encountered the Vilani and a few decades later wast TL12, 1 whole TL above the Vilani.
Simple explanation - the Vilani were a stable TL11 culture so the Terrans could rapidly beg, borrow and steal the necessary data. Thing is the Vilani had been TL11 for an awfully long time.
Discoveries and new science was culturally suppressed, but it would have still been there in the data bases and the universities etc.
Terran merchants, spies, exchange students, whatever, managed to get hold of the secret stuff and Terran scientists and engineers didn't have the same cultural blinkers to stop them developing TL12 stuff.
But then we have a problem - making the leap to TL13 requires brand new research with no clues, no secret data bases etc.
So what I think happened - tech stagnation on the developed, self sufficient worlds.
Tech regression and then progression back to TL11 on many Vilani worlds that could cope with the initial trade disruption.
There would have been riots, economic migration, refugees, civil wars, epidemics on worlds that couldn't cope, probably quite a lot on the worlds that could too. But eventually things would either calm down or the world would fail.