It may help to know a bit of history: KDE3 themes could include a bespoke widget style, and QT3 widget styles were always implemented as executables (you can look at modified versions of the C++ code in the TDE git repository, if you’re really bored). So keeping code out of the themes hasn’t been important to KDE for at least the past 20 years. If I’m not mistaken, far more things are stylable in current versions of KDE. That doesn’t mean that every theme will style all of them, though—you can have codeless styles like the one you found, that make use of the built-ins rather than trying to change All The Things.
It may help to know a bit of history: KDE3 themes could include a bespoke widget style, and QT3 widget styles were always implemented as executables (you can look at modified versions of the C++ code in the TDE git repository, if you’re really bored). So keeping code out of the themes hasn’t been important to KDE for at least the past 20 years. If I’m not mistaken, far more things are stylable in current versions of KDE. That doesn’t mean that every theme will style all of them, though—you can have codeless styles like the one you found, that make use of the built-ins rather than trying to change All The Things.