The second player could be off screen...? I mean, the camera is rather zoomed in, the stage is fairly decently sized, and there are platforms at the edge of the arena that the second character could be located at. So, if he is a boss, there's that against him being a playable character.
Maybe a slight glimmer of hope for Ridley supporters (I'm indifferent to the cause personally):
Watch the section from about 9:50 to 10:34 minutes (with or without sounds)
For those who don't see what I'm getting at, or those who don't wish to watch the video, here's what I mean: The video points out that we get our first look at the Pyrosphere's Final Destination form. It has no lava/magma. If Final Destination forms of stages remove stage hazards and there is no lava/magma, does that make it the stage hazard? If so, why would Ridley also be a stage hazard-boss if there is already a stage hazard?
Granted this is easily debunked. The aesthetic of lava/magma may be removed to be less distracting. Ridley may only show up when lava/magma is not being used as the hazard.
As far as I know, there has never been a stage with more than one very present stage hazard. I could be wrong though.