That's an unreasonable thing to do, especially since it was less an ad hominem attack than it was pointing out the fallacy at the root of your misunderstanding. They were doing you a favor. But mostly because, "I will take this one line from your statement, to show that you were arguing in bad faith, and use it to nullify the rest of what you said," is... very obviously a rhetorical tactic, rather than a substantive counterargument.