I think you're misinterpreting the article regarding production worker tasks. While they do suggest that production workers were involved - "By considering the workers who would have to put the water heater together—in fact, by having those workers right at the table, looking at the design as it was drawn" - they don't say that the production workers themselves designed any products, let alone from scratch. It's fairly clear that feedback from some production workers was taken into account during the design process, nothing more.
Mechanical design work of appliances is not something that you'd do alongside shift work. The workers getting paid $13.50 an hour are not going to be mech-eng degree holding designers.