And your bad comparison doesn't make for a good argument. Or a legitimate argument to begin with. Those are just helping and barely are part of the moveset, not a focus of it either. Pac-Man has quite a few cameos among his moveset. In fact, that's basically what he is, a walking arcade reference. It's the only reason he even became playable, as Sakurai couldn't figure out a way to implement him at all. But if you want to be actually accurate, Pac-Man's moveset is "Namco Arcade plus myself", while Duck Hunt is "Three characters in one, representing the Duck Hunt universe while using the NES Zapper player and other characters from the NES Zapper series". There's a difference between cameos(which is everyone who isn't Pac-Man among his moveset) and an actual character represented by the moveset(the NES Zapper player character). It's like saying the Pikmin aren't a major part of the moveset,or that Pokemon Trainer is irrelevant(which is proven that both are immensely important. It's why Charizard when separated not having a Trainer makes actual sense. It's because it's no longer Pokemon Trainer, who is a commander of three specific Pokemon. This is probably also why we don't see any Trainers for any other Pokemon, as it's a very strict character setup).
The difference here is that Duck Hunt is a trio of playable characters all intentionally working together(the NES Zapper user, the Dog, and the Duck).
Back in 1987, when Duck Hunt came out, these two might not have had the best relationship. That all seems to have changed now, and this unlikely pair are here to fight together. After throwing a can or a clay pigeon, press the button again to have a little helper fire the NES Zapper for extra damage!
Even the PAL version hilariously notes the third character, despite its inaccurate translation. Note that the other trophies simply don't talk about the NES Zapper character, so in context, them referring to just the Duck and the Dog as a duo makes sense. Doesn't mean the full character isn't a trio.