Well, nor was Marth in Awakening, but he is mentioned. Dark Meta Knight appears in Triple Deluxe, so that could be "close enough" for Sakurai. Not like being the real thing would matter at this point, right? Notably, his Dark/Blue costume in Brawl is based upon Dark Meta Knight's artwork, with some small differences.
So seeing as he got somewhat of a costume in Brawl, seeing him actually appear again in TD gives them hope that Sakurai will find this a reasonable enough appearance to announce him. It's still straw-grasping, but I can see the logic. The other is "It's a Kirby game, that should be enough. Hey, his clone appears, sure why not?" Because again, look at Marth coming back. Why attribute him to a game he doesn't appear in, but a disguise of him does? Yeah, spoilers, I guess. Just not heavy ones. ...Then again, I think we all know what I mean. If not, well, good, cause I won't elaborate outside of spoiler text.