This widget looks quite app-like when open in a windowed form using the "Windowed widgets" KRunner runner search result. When launched in that way, it can give the user the impression that it's a real app. This presents some issues:
If the user has a real calculator app installed, two calculator results will appear in search results, which is weird and confusing. People will wonder, "Why does KDE ship with two calculator apps"?
If the user has no calculator apps installed, this will appear in search results anyway and they will think it is the official KDE calculator app and use it instead of using the "download a calculator app" search result from the AppStream runner. The widget is quite simple with almost no features, which is fine for a widget. But if the user misunderstands this and thinks it's the official KDE calculator app, they'll be lacking those features and form an incorrect impression of KDE's ability to write good calculator apps, because in truth, we have two powerful high quality calculator apps (KCalc and Kalk).