Why not stall? Maybe he wants to troll. I mean he said Peach's moveset would change yet she's still the same down to the Final Smash.
Right, let's see.
First of all, Sakurai has been fairly consistent in explicitly deconfirming characters. Waluigi? PotD text said he is an assist trophy. Ashley? PotD text said she is an assist trophy. Ghirahim, Isabelle, Dark Samus, Midna, the list goes on and on. Recall Chrom? Some people were still convinced he was playable, despite being clearly shown in a final smash and having no banner on the website. What happened? An official statement that he is not, in fact, playable. Remember Prince Sablé? He was one the few not to be explicitly deconfirmed in th PotD, and people were getting their hopes up. What happened? A tweet confirming that yes, Sablé is an AT, not a PC.
Sakurai probably does this because
he doesn't want to create fake hype. He doesn't like it when tweets and the like (this includes PotDs) cause people to get hyped up when there really is no reason to do so because whatever they might be thinking is true isn't true in reality.
See what I am getting at?
Secondly, Ridley is part of the Big Three. He is one of the most requested characters in the west, having a very strong fanbase. Not only that, he also is flat out the most controversial character yet, bar none. It is known that the Sm4sh team is more involved with fans now, and also aware of what the fans want. They are also very well aware of Ridley's situation. Bill Trinen
cracked a joke about Ridley after being asked about him, and one of the Smashboard moderators even met an employee who, after said mod said something about Ridley being too big, responded with "Samus is too big". They know what we want. No, it isn't proof that we are getting it, but it might increase our chances ever so slightly.
Third, by first teasing Ridley's existence, a few months later show his shadow while leaving his role vague enough to be considered ambiguous, and then even months later not reveal anything about his true role, only for him to be a boss anyway, would be alienating a large part of the fanbase who put their trust into Sakurai finally delivering after 13 years of waiting. At that point it stops being trolling, and starts being bad business practice.
I talk too much.
EDIT: Also, of all the bosses to tease, why Ridley? Why the big purple space dragon with a gigantic fanbase, why not some other boss?