Game Freak didn't make Flying Pokemon immune to specific moves, they made it immune to the entire type. Take for instance the move Bone Club: it seems like it should be able to hit a Flying type, yet they are immune to that move.
It would probably be a bit too confusing if Game Freak opted to make Pokemon immune to specific moves, rather than overarching types of moves, hence why Hidden Power [Ground] does not affect Flying types.
Source: Reasoning
Hope I helped!