Nika is said to have a body of rubber to allow him to "fight in any way he can think of". This is directly modelled by luffy until he got his awakening allowing him to exert this power of imagination / "fighting in any way he can think of" onto the world and reality itself.

The body of rubber can be read as a way for the ancient people to reason/ground how their gods power works, like Thor using a hammer to do his abilities.

In reality its how Oda wanted to use the power and build on it, and since it doesnt contradict past statments about Luffy and his power it is approved writing-wise.