Regardless of magma "burning" fire or not, it was made perfectly clear that Akainu > Ace when they first clashed, seanos also makes an excellent point, if he jumped in to save Luffy disolving into fire before getting hit would acomplish nothing.
There was a part that i believe they skipped where Luffy tried to stand up and evade Akainu but was too tired to move, that`s when the paper dropped.Originally Posted by seanos