On Pokemon HG/SS, how do you fight these characters?

On youtube i saw a video of the player in Pokemon Heart Gold/Soul Silver fight Team Galactic Leader Cyrus, Hoen Champion Steven, and a couple other gym leaders from the previous games Pokemon Diamond, Pearl, and Platinum???? Is it a hacked thing?? Or is it a real thing in the game? Because is it really possible to fight them? Can anybody tell me about how the player got to fight those characters?