The fact that they increase zoned defense instead of regular defence means they intended the zone defence limits to apply. There is no other reason to include the "on all zones" text.Defense caps at 4, on Page 228, "The maximum amount of defense a ship or vehicle can have in any of its defense zones is four points, regardless of its size.”
On page 134, Defensive Driving "Any vehicle the character pilots has its defense rating on all zones increased by one per rank of Defensive Driving.".
Because this feature applies to all zones, and not just one.
As opposed to "Reinforced Shield Generator," for example, which specifies "to one defense zone of choice."