I'm saying that that Shanks slash cut the sky; WB parted it. Pure muscle did not create a fissure in the sky. Figure there is more to anything Shanks can do than pure muscle. Who thought Haki would turn Luffy's hands black?
No bubble for the same reason we saw no hint of arms in Robin's first scene. Muscle could maybe move or slice a cloud but distort the atmosphere so a void actually appears in the sky but not effect the ship or water?
Really don't see how it wasn't obvious when we first saw WB first crack the sky like glass just because Oda didn't draw a bubble. Not drawing arms seems a bigger inconsistency than that. We don't even know if people in OP actually see a bubble or if its just for the reader.