The punch didn't hit the strings. It hit the concept of the strings.
Luffy was no longer a man. He was a titan of obsidian rubber, his skin a sheen of armament Haki. His torso was a barrel of coiled muscle, his arms laced with intricate, tribal patterns of pure, hardened will. His hair flamed up, wreathed in the same steam as his body. He was half again as tall as Doflamingo, but his legs were lean, his feet tapping the ground with a rhythmic, terrifying thump-thump-thump . luffy gear 4 episode
Luffy’s fist, the size of a boulder and blacker than midnight, came down. The punch didn't hit the strings
" "
Then, the rubble stirred.