Haha, I experienced the exact same thing and even changed the voice of my post from
"This is clearly a mistake" to "Isn't this a mistake?"
But then again the sentence
…accessing those C++ classes directly from Swift isn’t possible, so a layer of C++ connects the two.
really made no sense to me.
As the C++ in this layer is
extern C it basically is C I'd say.
But yeah, I agree with you – it's not clear to me either if you'd consider the C++ function in a C layer when you annotate it that way or if you would still call it a C++ layer.