No, it's not the sky. That's not how the sky looks from a pit. The spiral path is the outline of the path Kaido's body took as it falls into the bottomless pit. BM was nowhere to be seen because we did not see Kaido hitting the bottom.
Well thats how the sky looks from a pit alright. The pit is pitch black while the sky is illuminated. If it's the outline of the path Kaidou's body took, its supposed to be dark since the inside of the hole is not illuminated.
If its the hole BM fell in, the outline will be round and much wider as the bomb exploded first.
If its BM's hole, Kaidou's entrance to the ground wont explode the entire surface as there is already a bigger hole in the first place, acting like a wide door for Kaidou's body to enter.
Also if you said it's the hole BM fell in, or you say that Kaidou didnt fall to its bottom, you can prove it instead since there is no proof that Luffy aimed for BM's hole exactly, no proof whatsoever since the outline directly opposed an outline of bomb explosion, etc.