I don't remember anything that could let us think that. Explain.
Replay a Lion at the door. It is said there that Shivans have a new cruiser, a new fighter class, but NEVER that they have new weapons (for example at the cain there).
In fact, at no place in the campaign is it ever said that the Shivans have a new weaponry like beam cannons or flak guns. Well for flak guns you can say they appear only on new ships (Moloch/Sath/Ravana), but the sudden appearance of beam cannons on demons, cains, lilith? Doesn't get mentioned at all.
And to quote from "The Great Hunt":
With the technology they had thirty years ago it's a mircale we won the great war.
In FS1 they didn't have different weaponry than the Terrans and Vasudans, only at the first few missions shields were new. Only the Lucifer was special.
And last but not least it's hard to believe the Shivans had no technological advancement in 8000 years, but suddenly a lot of it within 30 years.
Though this can be explained in other ways (for example the Shivans of FS1 and FS2 are different Shivans).