I thought the consensus at this point was that "windmill" basically meant "wind turbine", at least colloquially, in the same way that a "battery" really should be called a "cell".
I'd got the impression that "windmill" was the deliberately wrong use of terminology by people opposed to wind power to trivialise or ridicule it. But that may just be me reading the Telegraph.
In my particular case, it wasn't meant to be offensive or dismissive at all; I think wind turbines are very cool.