Most games say that on the cover (not just gamecube games, xbox games too). I think it's just a marketing trick with ambiguous wording.
In other words, they really mean that "the disc in this box will only run on gamecube", but they want you to think that "this game was only released on gamecube" because exclusives sell better on each console (even though when you add the sales from multiple consoles multiplaform games sell better).
On the other hand, it's possible they are including the extra disc with "Pacman Vs" as they did with another Pacman game. In this case you should definately get this game (since Pacman Vs is awesome).