-asura is ryou + seimei kikan: zoro is making his haki/demon aura take the shape of multiple arms and hrads and control them like they're parts of his body.
we have examples of people making their haki change form and nature, kinemon's fire, kaido/yamato's lightning, vista's flower petals...
seimei kikan is the ability to control all parts of the body, even parts that cannot be consciously controlled, and i think the same applies to thing created from devil fruits like doflamingo's doppelganger and katakuri's morphing ability and their awakening, or in this case zoro's additional body parts.
-diable jambe is a chemical reaction triggered by intense emotions, there's something is sanji's germa blood that reacts to his emotional state, and raises his body temperature, eventually making him combust into flames.
asura can be copied as it operates on basic concepts, just like how zoro easily copied kinemon's foxfire style.
diable jambe can't be replicated, because it's a bloodline specific ability, even though there're a lot of characters who can produce fire, it is still not the same thing as sanji.