If he is a swordsman - or even just extremely proficient with the sword - then yes I'd rate him on a list of best swordsmen.
I know King doesn't limit himself to being a swordsman but he is very skilled with the sword, so skilled in fact he is better with the sword than most swordsmen we have seen. I see no problem with ranking him in a list of swordsmen based on his skill with the sword, then you could also rank his strength overall in a separate list.
It's kind of like an MMA-fighter can be ranked as #3 in the world as a pure kickboxer when only allowed to use those techniques and #10 in the world overall as a MMA-fighter when all techniques are allowed.
I know Law ranks as a swordsman as seen in the picture Oda drew of One Piece swordsmen but to me he has hardly shown any swordsman skill. Some, sure, but in his case it's mostly DF. Same as Big Mom.
I believe Mihawk > Roger as a swordsman but in a fight using all their techniques Roger > Mihawk. So Nasujuro could be stronger or weaker than Mihawk overall, as a swordsman though Mihawk > all (Ryuma only possible exception).
We are about to witness Tyson Fury vs Francis Ngannou in real life this weekend. I'm 100 % sure Fury wins in boxing but in any other discipline like wrestling, BJJ, kickboxing or MMA Ngannou wins.