This vein attempt of defying what was clearly seen before our very eyes is going to lead you to some deep trouble.
Let's go over exactly what we saw:
1. A shadow in the shape of Ridley
2. Small portions of his left wing, which happen to be gray and straight-edged
3. The shadow stops mid-map, which can be compared to the actions of the Yellow Devil
From this, I would estimate that there is a gray-coloured boss with Ridley's shape on the Pyrosphere stage. My estimation is that the boss is incomplete, and those are just polygons. That said, it leaves three possibilities:
1. It's Meta-Ridley the entire time.
2. It's Ridley, but gets switched with Meta-Ridley when in battle.
3. Ridley's the boss, and that's that.
I think all are easy possibilities..
Edit: Now that I've seen for sure that the shadow was Other M Ridley, this is all worthless. I do, however, still have confidence in Ridley, for reasons you should see in my later posts.