His fruit is what is creating strings on the ground because his DF became part of the area. He could never just create strings out of rocks out of nowhere unless his DF transformed the ground to be able to make string just like it enabled his body to make strings when he ate it. It's not made up, it's the only sensible explanation.
Luffy won't turn the ground into rubber, the ground will "eat" the DF to become rubber like Luffy did, that's what awakening is. Your DF ability goes into your surroundings and lets you control it like an extension of your body.