I could write up a leak right now and predict the remaining roster. It would be a complete guess with no legit evidence. I just conjured it up in my head. Even if every single guess came true, my leak would still be fake. Accurate, but fake, and could be pure lucky guesswork.
I could be a person loosely involved with the game (an office janitor or something, I dunno), and maybe I sneak a peek of the design documents. I see some character images, and when I make my leak, I say Pac-Man is in the game, but also Bayonetta. However, Bayonetta never shows up. Turns out I mixed her up with Zero Suit Samus because of the heels and long hair. This leak may be inaccurate, but it'd still be real, as I actually saw official material in the game's development studio.
Sal Romano's leak only fits into the former category right now, he's not provided any proof that his tipper actually saw anything. However, the Mario Maker leak, was real, because it showed a photograph of a venue that matched up with the actual E3 hall, and Mario Maker was there.
This point has been re-iterated in many interviews, and Brawl was exactly the same situation. The latest instance was the round table from last night.
These two prove Pac-Man could have been planned even before Namco was on board, and could have potentially made it into the game regardless of their status as the dev team of the game.