It's for formal verification and validation (V&V).
> why the development of a simulation should hold up the full production
Why shouldn't it hold up production? The program has requirements for formal V&V that they haven't met yet. In theory, you could start production by accepting the risk that you might need to do some rework, but I don't know if that was even an option.
> step through iterations with very quickly
The argument for doing a simulation is to get higher fidelity. It doesn't really make the process any faster on its own. The simulations themselves take a trivial amount of time compared to the preparation and analysis.
> nothing like this has been done before
There's a lot of simulation in aerospace and defense. I don't know exactly how JSE compares in terms of extent and reliance on it, but it's not a completely new idea.