Confirmed via Tweet - 343i Community Manager, Josh Junyszek (@Unyshek) said “Equipment is placed on the map for pickup” when asked about the grappling hook. This means it’s not a default ability such as sprint, slide, and clamber but a piece of equipment such as the bubble shield and power drainer.
I also noticed in the gameplay montage at the 28 second mark (the grappling hook/no scope), there is an illuminated spot on the floor. The player runs to that mark then looks to the ceiling, where there seems to be another illuminated spot. The player aims at it and releases the grappling hook sending him flying into the air and landing a dirty dirty no-scope.
Looks like it could potentially be a form of map mobility such as the man cannon and lifts?
Check it out!