I don't remember it that way. I took it more more as being about the definition of "fish", and demonstrating that whalers don't think about things the same way that non-whalers do. It's not ignorance; he acknowledges repeatedly throughout that whales breathe air, and nurse their young, and other mammalian traits.

From a phylogenetic perspective, whales are fish.

So are we.

not a counterfactual. "fish" was a legal definition which allowed hunting.

yeah, they get called fish (something that swims, like modern military torpedo), but the whalers knew they were not