400W x 3 hrs = 1.2 kWh = 1.2% of a 100kWh battery.
But that was only one of the issues. They also mentioned waste heat and being able to retrofit the computer to older cars as reasons to keep the power down.
If you're driving a Tesla for three hours, you have likely drained a huge chunk of the battery already, either via the traction motors alone, or in combination with climate control and waste from braking. Tesla air conditioning alone can draw 600-2000 watts. Inference power draw is really nothing.
But that was only one of the issues. They also mentioned waste heat and being able to retrofit the computer to older cars as reasons to keep the power down.