A tornado, as someone else put it, is a razor blade cutting across the landscape. The best predictor of whether or not you'll survive a tornado is if you're in that very thin path of destruction. And that path is not known until very, very short time horizons before the event.
As far as a manager knows, the factory is as risky a place to be in a tornado as wherever the workers' homes are. Actually, you can make some cases for the factory being less risky than homes: if the home is a trailer park, well, that is almost certainly toast in a tornado; additionally, workers are going to be awake at the factory versus at home, and are therefore more able to respond to a tornado siren going off.
(This does make some assumptions about the factory having appropriate tornado shelter, and having reasonable policies around what to do in event of tornado warning. I don't have enough information to know if those assumptions hold, but if they don't hold, I would ABSOLUTELY castigate the company for failing those assumptions.)