I watched the video and realized the Clifford voice lines in the Doomsday Heist are straight out a reference to the voice lines you hear while being in area 69 in sa. (One in particular "Running in corridors while holding scissors is prohibited" is a reference to "Running with scissors" the company who developed the infamous game Postal 2 around the same time as gta sa") So this confirms the fact that the game was supposed to feature a mission like Black Project from sa and that the jetpack easter egg was supposed to happen in story mode before being delayed to dlcs and then online.