"그는 물고기처럼 수영하지만 물고기라고 말할 수는 없습니다."
번역:He swims like a fish but it is not necessarily a fish.
I have noticed this sentence in the other courses teaching English so it is not likely to be changed anytime soon. As a native English speaker I would assert that the base sentence is pretty strange. In particular, the use of "it" makes it awkward and put into question whether "it" refers to him or to something mentioned previously in a hypothetical earlier part of the conversation. I would suggest something along the lines of, "While he may swim like a fish, he isn't necessarily one" or "While he may swim like a fish, he is still a man after all."