Edit: let me preemptively be a pedant to myself and say that "sub-...-species" is wrong because "bilaterally symmetrical animals" is not a species. Flounder is itself a species AFAIK, not a sub-species of anything. It is a descendant of the common ancestor of all bilaterally symmetrical animals. There, now surely no one will find anything to be pedantic about :D
I appreciate that information. However, flounders themselves are not bilaterally symmetrical. I have caught many dozens of them and it's pretty easy to tell that they are not.
Isn't it referring to during development? Like as they're forming, they are bilateral? I haven't taken developmental biology in many years, so I'm maybe wrong.
They're only bilateral when they're very young. And even then, everyone is just focusing on the eyes. The body of the fish is also not exactly bilateral. Just fillet a flounder of any age (or watch a video on it) and you'll see.