Lens sharpness tool is based on a specific calibration made for each lens-camera combination. And for each given lens-camera combination, we measure variation of sharpness across image field, for each combination of focal and aperture. Consequently, as diffraction contributes more and more to lens sharpness when aperture decreases, its effect is taken into account in our dxo modules, and thus lens sharpness tool has an impact on it.
This said, when diffraction is strong (or more generally when input lens sharpness is very poor), our dxo modules limit their correction by default, as fully correcting the picture might lead to artifacts (and when a small detail is very blurry, there’s not that much to do to recover it…). Nonetheless, adjusting manually lens sharpness sliders enables to increase again sharpness.