Well, a disease, disorder, or syndrome aren’t necessarily exactly the same thing. They often get used interchangeably on a colloquial level, but they do have specific usages in more narrow usage.

Williams, or Williams-Beuren, syndrome (like all syndromes in a medical sense) is really just a cluster of symptoms. Since many of those symptoms are negative in their effects on overall health, it can’t be considered a net positive the way your question asks.

That being said, there is also still room for debate if the kind of behavioral and social specific traits that come along with that are actually desirable for human society.

The heightened empathy would sure be nice most of the time, but it also has its drawbacks since empathy can also make difficult decisions much harder to both make and implement. I’m not going to weigh in on that on a personal level because I’m not really sure exactly where I stand with it internally.

So, I think the answer to your question is that we classify the syndrome as a disease or disorder instead of being a desirable and/or beneficial thing because of the other associated symptoms. Kinda hard to wish all the sensory issues, heart troubles, higher incidence of diabetes, etc as a good thing when the only real benefit of the set of genetic changes is a reduction in social fear and an associated bump to empathy. Waaaay more drawbacks than benefits