This almost certainly has to be the case. There's no way they could've went from drawing board to working prototype in less than a year. Not on a ship that large and full of new technology.
Why not? They went from a transporter equation to a portable transporter unit that can send you from one solar system to another in the same amount of time.
Whole different ballgame. You're talking about building once piece of tech, that's really nothing much more than an upgrade to existing tech. Versus funding, then building an entire ship employing new engines, computers, flight dynamics, and weapons. Then make all the new stuff work and then work together, manufacture it, then put it all together on a newly built spaceframe.
Vengance's class, or at least the project that led to her, was probably not secret. What the ship could really do or was meant to do probably was.