That scene is not cannon at all I guess. His model and animations may not have been over at that time (if the Wii U version has no release date yet, there's a reason to that, it needs more time). So they just showed his shadow floating there.
Well, the only thing that can appear at any second specifically in a stage is a stage hazard/boss. So yeah, there's a boss in Pyrosphere. And please, it would so dumb not to have Ridley as a boss here. Fighting Ridley on Pyrsosphere makes perfect sense.
If it was just something in the background, there would be no point in saying that.
It's true that the scene will have been heavily scripted in order to show the shadow chasing Pikachu like that, but there is absolutely no reason for Ridley to be incomplete. As I said before, they wouldn't tease him while he was incomplete. It'd be pointless, as they could just wait for the next Direct and could tease him while complete. He didn't
have to be at the end of the Yellow Devil section. It's highly unlikely they would put him on the backburner after starting development and then show him as unfinished.
You realise that if the scene isn't canon, it doesn't really injure Ridley's chances, correct? If the scene isn't canon, it means they're definitely hiding something. To have him as a boss and to create a scene that isn't even a part of the game to showcase said boss in a purposely ambiguous way would allow for false hype that would be crashed down at the end, which Sakurai doesn't like creating. If Ridley was a boss, there would be nothing to lose just showing him. It would still count as a tease for people that want him to be a boss, and would have put to rest supporters, clearly disconfirming him.
That's just outright not true! You can make any character appear on a stage by selecting them, assist trophies appear at any second, Pokéballs... to claim that it could only mean a stage boss is outright
wrong, regardless of implications. In hindsight after the Aprril Direct it may seem to imply stage boss, but before the Direct it was as everyone knows the line is; ambiguous as hell. There is not necessarily a boss on Pyrosphere at all.
It being dumb to not have Ridley as a boss is completely down to your opinion. For example, I can argue back "I don't think so at all. The Pyrosphere doesn't always have Ridley on it, and just because the Pyrosphere is there doesn't mean Ridley has to be" and it's just as valid a point as yours.
It's a collective consensus that Ridley is either playable or a boss. I believe that he could be either, but people that say he's definitely one or the other are looking through it with tinted glasses. There's enough ambiguity here for it to go either way, and if you think he's definitely, 100%, without room for doubt a boss, you're completely and totally gullible and are probably the type of person that had their blood turn stone cold in shock when Palutena was revealed to be playable after the PotD tease.
If you think he's 100% playable, keep in mind that it's entirely possible that Sakurai could have him as a boss just to showcase the new stage boss gimmick. Absolutely nothing is certain until he is confirmed or disconfirmed.