They wouldn't have to hire 20 extra managers, they just would need to develop some materials and train the managers they have.

And social engineering 1000 managers is FAR more work than scaling a software hack across 1000 machines, and also far less likely to succeed. At least one of those managers is going to get suspicious and call corporate first. Moreover, it will be pretty hard to get an on-site manager looking right at the full freezers to set them to defrost....