Windows Unicode strings happen to fit this definition, but of course, so would UTF-8 strings. Either way, in Windows's version of reality the definition is narrowed to mean "potentially ill-formed UTF-16LE encoded string", but that doesn't exactly roll off the tongue, so for better or for worse, "Unicode string" is the consistently and widely used Windows-specific nomenclature.
My post specifically mentions UTF-16 and WTF-16, too. The term "Unicode string" exists and is well established: https://unicode.org/glossary/#unicode_string.
Windows Unicode strings happen to fit this definition, but of course, so would UTF-8 strings. Either way, in Windows's version of reality the definition is narrowed to mean "potentially ill-formed UTF-16LE encoded string", but that doesn't exactly roll off the tongue, so for better or for worse, "Unicode string" is the consistently and widely used Windows-specific nomenclature.