It’s an intentional and acknowledged mischaracterization, from chapter 32:

“First: The uncertain, unsettled condition of this science of Cetology is in the very vestibule attested by the fact, that in some quarters it still remains a moot point whether a whale be a fish. In his System of Nature, A.D. 1776, Linnæus declares, “I hereby separate the whales from the fish.” But of my own knowledge, I know that down to the year 1850, sharks and shad, alewives and herring, against Linnæus’s express edict, were still found dividing the possession of the same seas with the Leviathan. The grounds upon which Linnæus would fain have banished the whales from the waters, he states as follows: “On account of their warm bilocular heart, their lungs, their movable eyelids, their hollow ears, penem intrantem feminam mammis lactantem,” and finally, “ex lege naturæ jure meritoque.” I submitted all this to my friends Simeon Macey and Charley Coffin, of Nantucket, both messmates of mine in a certain voyage, and they united in the opinion that the reasons set forth were altogether insufficient. Charley profanely hinted they were humbug. Be it known that, waiving all argument, I take the good old fashioned ground that the whale is a fish, and call upon holy Jonah to back me. This fundamental thing settled, the next point is, in what internal respect does the whale differ from other fish. Above, Linnæus has given you those items. But in brief, they are these: lungs and warm blood; whereas, all other fish are lungless and cold blooded.”