The documentation says:
QTouchEvent delivery is independent from that of QMouseEvent. On some windowing systems, mouse events are also sent for the primary touch point. This means it is possible for your widget to receive both QTouchEvent and QMouseEvent for the same user interaction point. You can use the QTouchEvent::TouchPoint::isPrimary() function to identify the primary touch point.
But this leaves unanswered how to ignore the mouse events that are going to also be delivered as touch events. Imagine a widget that needs to respond to touch events when available, but fall back to mouse events otherwise.
This may just be a lack of documentation.