> the implementation has started from the factory, not from the world

This sounds plausible, but I'm not sure if it's entirely true: the factory building is explicitly set in a foreign world and not in our on as any of the many many other factory sims out there.

I think it is more like the authors had the "fight against xenos with factory" idea as a start and then got severely detoured into the factory building after they had the basics of the world done.