I doubt he meant it literally, probably more like he had to slow uncomfortably fast because the self driving car behaved in a somewhat jerky manner. Think about what you might do if you had to slow down in this situation -- you'd try to balance leaving room ahead of you as you did it with not stopping so quickly as to startle the driver behind you. There's a lot of nuance to that, and it wouldn't surprise me if computers weren't very good at it yet.