Let me try to be a bit more precise. Xander said something that you and I both agree was "a" truth. But having Xander say it does not make it true.. since Xander is certainly capable of misjudging people or situations. He's not some oracle. Indeed, while the writers might well believe that Buffy is emotionally closed off, there is no way for them to tell us that in the text in a way that says "even if you don't agree, this is true about Buffy". They've either shown us a Buffy who is closed off, which would lead us to agree with Xander, or they haven't. All they can have meant by having Xander say it is that Xander sees her that way or at least wants Buffy to think he sees her that way. Whether we buy it or not is up to us.