Check the Codex, this is actually true. Council races salvaged Sovereign's weapons and made their versions, like the thannix cannon (you use it at the end of ME2 if you upgrade, one shots enemy ships).
So yeah... maybe they didn't upgrade all that much between 2 and 3, but their guns are much better than in ME1.
Funny, when I wrote the above post I was considering adding "Or maybe the council did decide to adapt sovereigns tech, despite "him" not existing." Guess I was right.