Dark Pit was acknowledged as his own separate character in the Palutena reveal, so I highly doubt that he'll be an alternate costume for Pit.
Doesn't mean he can't be an alternate costume as a pseudo-character. Sakurai confirmed that Lucina was originally meant to be an alternate costume for Marth, so its conceivable that she could have had her own name and voice along with her model.
Dark Pit could easily be the same.
You are right! All the Alts that have been announced so far (Wire Frame Mac, Male Wii Fit Trainer, Female Villagers, and Female Robin) were not in an actual trailer. Theybwere just confirmed by Sakurai. If Dark Pit was an Alt I believe Sakurai would of BEEN confirmed that. He doesn't believe in false hype!
WireFrame Mac is actually just a texture swap I think, since it doesn't actually change the model at all, his actual alternate costume is his Pink Sweat Suit, which did appear in his Trailer at the beginning.
Male Wii Fit Trainer, Female Villager, and Female Robin didn't need to appear in their respective trailers simply because they are meant to be the same character, just different genders..though Female Robin was kind of tacked onto that trailer. Its also possible they didn't have costumes ready when those trailers were created.
As for Dark Pit, he's his own established character separate from Pit.
Also, Sakurai hasn't always been one to confirm something right away. He shown off a Pic of the Day showing off both Female and Male Wii Fit Trainers half a year before revealing Male Wii Fit Trainer as a costume during the Smash Bros. Direct and then there's the Ridley Trolling he's still doing a full year after first teasing Ridley as a boss but not outright confirming it.
Dark Pit could still be a character, just with his own Name and Voice similar to how Lucina would have had her own voice and the Male/Female WFTs and Robin have their own voices. He would be like a pseudo-character.
I for one am hoping this leak is real, and playing as the Duck Hunt Dog sounds tons of fun. Also for the people who KEEP saying that Dr. Mario wouldn't be a character just an alt costume for Mario, remember that Sakurai said that if different characters have the same move set but play differently they get their own spot on the roster. Same goes for Dark Pit, if he deals damage differently he gets his own spot.
Dr. Mario would be better off as a costume for Mario though, its still Mario. It honestly always bugged me that Mario & Dr. Mario had different stats in Melee. It didn't make sense, they are literally the same character just in a different costume, why would his stats be different? Project M also shown that it could be possible to change a Fireball into a MegaVitamin if they wanted and, honestly, Dr. Mario with the Fludd isn't really that odd a thing since he IS Mario still.
As for Dark Pit, his stats being different from Pit's wouldn't make sense either, he is a literal clone of Pit and when you play as Dark Pit in Kid Icarus Uprising, he is a literal palette swap of Pit and acts exactly the same.
Can they be their own characters, sure, but anyone could be..we could have OoT and TP Link be their own separate characters with the same stats and moves if we wanted to, but it doesn't mean we should..