It says:

The gesture recognizer will only trigger on touch events. Specifically it will react on single touch points when using a touch screen and dual touch points when using a touchpad.

But this doesn't seem to be the true on at least Windows, Wayland and X11. QScroller uses QFlickGestureRecognizer under the hood which in turn handles only QEvent::Touch

(and mouse move events with a pressed button what is unrelated to TouchGesture) while touchpads send QWheelEvent. On Wayland touchpads send QWheelEvent with a scroll phase defined while on Windows and X11 they don't provide scroll phase and are indistinguishable from mouse scroll events.

This causes a prolonged confusion as could be seen from QTBUG-84588 and https://www.qtcentre.org/threads/55778-Scrolling-using-QScroller-class.