It's either that or oda came up with the durability thing later in the story, Marco doesn't have as much AP as zoro so it doesn't make sense for him to be able to damage flamme on king
Zoro and Marco are different combatants fundamentally, Marco uses brute attacks and zoro uses sword slashes. Why can't you use this as the basis of the argument instead..
a Brute attack had King downed..
Sword slashes of favorable Ap to King just get sealed off and have minimal affect like shishi sonson
and given match up it seems logical
rather than deeming it a retcon or an after though which is damn unlikely you can use the components given and reason with them ie what's more affective against and opponent and what not
so Marco downing King in his defence mode portrays that King's durability ie his flames are not affective against brute attacks
its simple you dont have to seem something a retcon or a discrepancy when they shouldn't be.