Ive never pulled it off against actual players, but have pulled it off against level 9s. Im sure you cannot actually get like 30%+ to death with that tho, but a few times are viable imo. It was mainly their to show the tech skill... not sure if any of those combos actually work or not. But Kirby's rock is really good imo, no one really uses it for punishes, it can easily be comboed into certain things like hammer at mid-high %s. Pretend the ones on Fox and DK only was the last footstool rock, that is an actual combo im sure since I get that on people and computers all the time.
It works on distant planet, the hazard stage part of Halberd, and battlefield... but the battlefield one is way hard to pull off while the other ones I can do pretty consistently. Its a Brawl tech that a lot of Kirbies discovered theorizing that it happens if the stage has like some sort of lip on it... im not exactly sure if its true or not. It might work on all stages, im just not sure.