I'm jumping on what some might call denial train, but I think it's still very possible that this is just another troll.
Anyone who has been following Sakurai's reveals over the past three games knows how much he loves to troll and mislead us about everything he can.
This looks to me like it may be more classic Sakurai.
He said that there will be bosses on other stages.... while showing (assumable) Ridley's shadow. There is no explicit link between Ridley's shadow and the boss on the Pyrosphere stage. Only an implicit link.
He knows how demanded Ridley is for this game, so it seems to me like he may have just picked him out to mess with us.
He has obviously purposely made everything about the Pyrosphere boss extremely ambiguous and mysterious. Why would he do that unless he was trolling?
I'd say Ridley still has a very reasonable chance. That shadow we saw may just have been a playable Ridley...
I'm a little surprised everyone is jumping ship on Ridley right now. He's been teasing Ridley and Pyrosphere forever, and yet again he avoided outright showing Ridley himself. If he's just a stage hazard/boss why not just get it out of the way already? Seemed to me it's just more trolling from Sakurai and that there's still more to the story, else he would have straight deconfirmed him like so many other characters today.
This.
I dunno how my title is wrong. Whether or not Ridley appears in the game in some other capacity, he has still been confirmed to appear on the Pyrosphere stage as a "boss".
He has not been confirmed to appear anywhere as anything.
All we've seen is a bunch of mysterious hints and shadows, which Sakurai clearly only revealed to confuse us and cause controversy. Otherwise he would have made a more explicit reveal.