The essay starts off being about whether or not whales are a kind of fish (and if you think that's an easy question to answer you really need to read the essay), wanders through border disputes and LGBT issues, and ends up being a philosophical treatise on why systems of categorization are never "wrong", only more or less useful.
In our present discussion, the question of "who qualifies as a terrorist" therefore cannot usefully revolve around an assertion that the BBC interviewee is wrong about his definition of terrorist. The speaker of the quote "Most Muslims aren't terrorists but most terrorists are Muslims..." is using a widely popular categorization and it's not helping to argue that his categorization is "wrong".
As to whether or not this popular categorization is useful for this investigation... I'm not sure.
(*) I believe I'm indebted to bluegargantua for the original pointer.