Actually I think there is a difference if you remember cp9 said the MOONWALK technique causes them to become light as a feather making them capable of bouncing on air but if you notice anyone that uses this Rokushiki tachnique has to continously stay bouncing but, the SKYWALK technique that Sanji uses him seems different he has the ability to stay a float ...
It's not that Kaido can not die, it's that he's so strong and so powerful that it's not enough to kill him.
That's why he has the title of the strongest man in the world.
Also, there is a theory that Kaido is actually a creation of Dr. Vegapunk and his body is made of steel, giving him immense strength.
Yes, it is a retelling of the Alabasta arc.
From wikipedia it is "adapting a story arc from the original manga" which is of course the same arc adapted for the anime.
There are some One Piece movies or specials like this one that can be useful for rewatching or catching up on the story faster than watching the anime or reading the manga.
The other main ...