The point wasn't that a game couldn't be fun if the story sucks; it's that gameplay and storytelling are not entirely separable (or at least, they shouldn't be; if they are, your game is probably an inelegant piece of shit produced by dropping a video game and a film into a blender).
On that note, I don't agree that Halo 3 is an entirely appropriate example of a game with great gameplay and poor storytelling. It has some spots of poor spoken dialogue, and it doesn't follow up well on Halo 2's villain setups (although where Truth is concerned I'm inclined to drop some of the blame on Halo 2 for setting up an act that couldn't be followed).
But the gameplay, plot, visual art, and music do an okay job of working together to produce a reasonably pleasing narrative structure, which is backed by some excellent vaguely-parallel terminals. The act structure is a bit lopsided, and as I already noted there are some ways in which it doesn't live up to it's potential, but overall I actually think Halo 3's storytelling is decent. Not in a way which will score well on a rubric that has you ranking plot and characters in their complexity as isolated entities, but in a way that can produce a good experience.
Err, are you expecting someone to call you out for changing your opinions? Are you expecting people to be shocked that someone who thinks Halo 4 is the best would also think that Halo 3 is the worst, or some such business?