This is 100% real, anybody can go to that spot in the code and see it. This guy didn't add it himself. Looks like the jetpack is being added.
More stuff is
DRIVEBY_JETPACK // Setting it so you can shoot from the JETPACK just like parachute
.?AVCGadgetJetpack@@ // the actual jetpack just like parachute.
Either that whole chiliad mystery was just leading up to DLC, or they purposely kept it out of the game so we couldn't find it in the files, and as everything in the game is tracked they added it into a patch when people got close enough.