Actually, I believe that the informant Sal got his information from was legit. To an extent.
Think about it; he was able to accurately claim "Animal Crossing Guy" a.k.a. Villager, Mega Man, and Wii Fit Trainer would be at E3 2013.
As for Little Mac, Miis, and Pac-Man, who all didn't show up at E3 yet have all been shown at a later time?
Legitimately lucky guesses.
My theory is that the informant actually had information in regards to the E3 2013 reveals. Specifically Villager, Mega Man, and Wii Fit Trainer. However, for whatever reason, the informant mentioned three other characters that would NOT be present.
It could be that if the informant mentioned only the ones that would actually show, it'd be much more obvious for Nintendo to find him out. It could be that the informant wanted to come off as a prophet using the characters that actually show as leverage. It could be that the informant thought that the other three were locks and included them in his message to Sal. Who knows?
Whatever the reason, it was coincidence that Miis, Pac-Man, and Little Mac were all in the game.
Now, what about the other parts of the leak, you may ask.
Well, the characters in question were quite easier to speculate about than the previous set with the sole exception of the Chorus Kids. And to a smaller extent the "Pokémon from X/Y". Not that it wasn't relatively easy to speculate we'd get a Pokémon from X/Y, but the common belief was that we were getting Brawl's Pokémon plus Mewtwo.
So my theory is that this ties into the prophet idea. With relatively safe bets plus the three accurate E3 characters, he could be seen as a legitimate prophet.
Then we get to E3 2014. Again, the informant got a hold of some legitimate info. Hence the name "Mii Fighters" as opposed to just "Miis" like before. Strangely, though, the informant only said that the remaining characters would be participating in the game and didn't raise specifics on to which ones would appear at E3. Though I have a feeling that the informant did in fact know at least Mii Fighters and Palutena would show up, given again the fact he used the term "Mii Fighters", and that he tried to emphasize Palutena as "Goddess Palutena". Given the circumstances of Pac-Man's debut, I'm unsure whether he could have really known beforehand.
Now of course, this is just my theory. I'm willing to buy the "I saw 'Protagonist of Fire Emblem Awakening' and assumed it meant Chrom" story and the "I mistook Robin for Shulk for Monday's reveal" story the informant is giving if the Chorus Kids actually do show up, though. Until they do, it just comes off as desperate backpeddling to me.