Well, the reason I think being directly named is relevant is because of how past characters have ended up in the game. For starters, Sakurai never mentions newcomers by their actual names before they are revealed. Usually this is just him not referencing the unrevealed newcomers at all, but there was one instance when Palutena's Temple was revealed that he referred to Palutena as the "Goddess of Light." It's clear that he was referring to Palutena, but he did not mention her by her actual name. Similarly, if you believe that Ridley is playable, a similar situation has happened with him; when the Pyrosphere was revealed, he was alluded to as "an enemy from Samus' past," and then when his shadow was shown in the Direct, Sakurai didn't acknowledge Ridley at all.
On the other end of the spectrum, characters that have been mentioned by name before actually being shown in the game have not been made playable. This can be seen with most de-confirmations, but the best example I can think of is when Sakurai mentioned the name Viridi with a picture of the Reset Bomb Forest stage. Some time later, she was shown off as a background detail.
So from what we've seen so far, if a character is playable, Sakurai will avoid referring to them by their actual name. Which makes sense, I think. Wouldn't want to go drawing attention to unrevealed characters. So in the case of Dark Pit, the fact that he was clearly shown at the end of Palutena's trailer, mentioned by name, and not actually confirmed leads me to believe that he's not playable.