I don't think the poster ever actually said that fish isn't meat, and I guess a lot of people are interpreting your question as a hostile challenge of the idea of pescatarianism (along the lines of "Why don't you think that cheeseburgers are murder?").

If you are struggling to understand pescatarianism as an idea, my perspective is that flexitarianism, pescatarianism, vegetarianism, and flexiveganism are all compromise diets that get some but not all environmental and ethical benefits of veganism. Different people have different levels of commitment and make different tradeoffs in their life choices. Whether that's "the only meat I eat is fish" or "I eat fish but not meat" is just a matter of definitions.

In terms of the English word "meat", there is certainly some ambiguity. At least as I know the term, it feels like a stretch (but maybe technically correct) to count fish as meat. I've had people suggest sushi when I say that I'm vegetarian, unaware that vegetarians don't eat fish. If you google for the Food Pyramid, most diagrams call out meat and fish as different things. "Vegetarian" almost always doesn't allow fish, but "meat" just isn't a very precise word.

Just adding the information contained in your second sentence here would have been enough to ask the question better. This kind of point is one that people typically get very hostile about, very quickly, in internet discussions, so if you're going to go there, you need to differentiate your comment from flamebait.

Also, scott_s's original comment didn't say that fish isn't meat, or even imply that. In English the word "meat" didn't traditionally include fish, and that usage is more than enough to cover what he probably meant. Taking the strongest plausible interpretation, as the guidelines request, makes the question unnecessary.