Some internet comments said this was for detecting people in the cabin, not for detecting things outside the car.
replyThey're not reading the actual FCC document then, which says:
replyStrategy: The move brings Tesla's sensor approach closer to competitors like Ford, GM, and Rivian, who utilize multi-modal systems (cameras plus radar) for their driver-assistance features.
Potential: This 'HD radar' could provide critical redundancy and data needed for achieving higher levels of driving automation and improving system performance in all conditions.