All great swords have a special ability/ element in them.
We saw guys using elements pts, brook and kin ats and zoro after tamed enma have that green underworld aura.
Shanks using some kind of special fire with his sword, only strengthen the argument that he is a swordsman and a damn good one because it seems his sword is one of the 12 supreme blade's.
Enma is probably the strongest of the great swords but given how hard was for zoro to master it, i can imagine it would be more difficult to master a supreme blade. If we take all that into account, shanks seems like a master swordsman not just a swordsman.
So you are kinda right, shanks is not just a swordsman....
It's not in my style but this is an absolute now