I don't understand what's actually wrong with this theory. Ifrit jambe has lightning coming out of it, which by definition is plasma. It probably is hotter than magma, however this doesn't mean that ifrit jambe is comparable to akainu's attacks.
The nature of akainu's punches go beyond the scope of normal magma. I'm not scientist, but I don't think magma can just dissolve you like akainu's fist does. imo Akainu's magma is either supernatural in nature, or he's combining it with internal destruction haki.