All of your points revolves around single point i.e. it took him some time hence it was high dif. Strength is the main factor that should decide the difficulty of a battle.
It's not just that it took him a long time, but why it took so long. If you have the easiest math problem in the world but took a long time to solve it because you were unfamiliar with the concept, then it was difficult to you.
I stopped reading at where I cut off the quote because I disagree entirely, but I can see how you can have that mindset. Intelligence can, at times, play just as big as a role. The fact of the matter is, if this fight was redone with the only factor on Zoro's side being that he's hacking and slicing, using just his muscles and not also his brain, Riku and Usopp would be dead and Zoro wouldn't have landed a single hit on Pica.