Regarding the Fall of Tinath, the Essaray don't strike me as higher tech than the Imperium. The biggest thing is that they were facing off against a bunch of small, rather isolated polities. Their tech appeared to be TL12 and all their troops and machines are so stupid.There are definitely polities of higher TL than the Imperium out beyond the borders of charted space - canonical fact.
The Vilani encountered them
the original owners of the Annic Nova encountered them
they pop up in Agent of the Imperium as the potential source of some of the threats
Fall of Tinath (best thing to happen in the OTU for a long time IMHO)
The first starfarers sidebar in MT
I personally have been working a lot on the Abyssals...
In addition, the danger jump space has on the uploaded minds of the directing Essaray leaders slows the danger because it seems like they have to get to places via sublight travel.
And who were the original owners of the Annic Nova?