It's not at all obvious that it's not, either. At the very least, it's against the spirit of the order.

The county order prohibits both businesses operating and individuals engaging in out-of-home activity except as explicitly permitted by the order, and violating a county public health order is a crime (misdemeanor) punishable by up jail.

Automobile manufacturing is not one of the listed exceptions, so it is illegal both to operate (for the owner) and to go to work at (for the employees) such a business.