You're right, Tesla doesn't have a 1mm steel sheet beneath the battery - it has a 5mm thick sheet [1]:
"Musk explained that damage to the armor plating on the bottom of the vehicle was inflicted by contact with debris of a specific shape that resulted in a lever action, puncturing the bottom of the Model S. He noted that 25 tons of force would be required to make the three-inch-deep puncture in the quarter-inch-thick armor plating."