Her vocabulary skills leave much to be desired. I think that she was searching for a word that word work as an insult, without sounding guttersnipey, but she failed to find a suitable one, is all.
Parsing that out, she said "you are being obtuse." She did not say, "your reply is being obtuse." If you just substitute the word "stupid" for "obtuse," in the first expression, there is no syntactic and no semantic error. Obviously, if your point is that words by themselves cannot be "obtuse" per se, I would say that was a debatable point. But her unambiguous use of “you are” rather than “your reply is” saves us from having that debate, because it clearly indicates she is imputing "obtuseness" to the person who wrote the reply, which is an indisputably correct usage. Are we there yet?