I've found many recipies that used materials from another craft. Just because it uses mythril doesnt mean it has to be goldsmithing, look up mythil axe, its still a smithing craft. i'm almost positive that all crafted axes, besides the ones enhanced by alchemy, are smithed, regardless of the ingredients. Most of the Polearm weapons are the same, theyre woodworking skills regardless of the ingredients.
It could be goldsmithing but it may very well be smithing. There are recipes that use mythril but are in fact smithing (its also listed as smithing here). I'll have to check up on this and see if I can synth it at lvl 40 goldsmithing.
So, it's an upgrade of the original... I'm guessing that it's a level 65 cap Goldsmithing recipe calling for a Combat Caster's Axe and a Mythril Ingot, synthed with a fire crystal.