While the First SW cause a huge amount of infrastructural damage, the minds behind creating and dispersing new equipment were still around - the lostech era wouldn't really begin until the near the end Second SW.
I have two thoughts on how the Seeker in particular was propagated:
First, the design was a Star League or Hegemony creation, but it never made it into production - the Star League preferred much larger transports as they operated in a bulk not seen even during the First SW. The design was stored in multiple locations as it wasn't considered particularly secretive or ground-breaking, so the taking of Hegemony worlds allowed multiple successor states to take and then build the design.
Second, the Seeker is actually multiple different classes based on a very similar role and chassis - all mostly independently created. As the First SW wore on, a role for a fast deployment of forces was found and this design fit the criteria. Eventually the design actually merged into a single class as the intelligence operations shared design specs from the different states until they were all virtually identical.
As for what happened in canon? *shrug*
