Well you are allowed a few steps on the ground for the Skywalk, I forget exactly how many but it is entirely possible that you just got lucky and did the challenge without even realizing it by zipping around with your flow motion!
Certainly does seem like you got the full badge to me.
I think I had something similar happen with the Aerial challenge when I got some really good combos off and it gave me the badge before I had even unlocked it.
So yeah looks like either you glitched it or just had a really chance run in Corona. I don't know what you mean by a green light to start the challenge, though. When I did it I just jumped off the tower, no setup or triggering to start it.
Edit: Also apparently Ultimate form doesn't count as "walking" because Sora naturally hovers above the ground. So if you were using that Keyblade at all from your previous runs, that might have contributed to it?
I don't have Ultima, so I must have gotten really really lucky... Even without the EZ Merit, that's the kind of silly challenge I like to try without being asked to...
Like, while playing Olympus, I suddenly wondered if it was possible to go all the way UP Thebes on Goofy's shield (and I did it! until I was stopped at the foot of one last wall, so at least I know it's possible to slide
down the whole city

I wonder why that's not a hidden trophy or something. It's so dumb and easy, and so much fun to do.)
About the set-up, I think I just misunderstood what someone wrote in a guide online...