From Generating Marker Boxes,

The list-item keyword causes the element to generate a ::marker pseudo-element box

::marker is a pseudo-element which may generate a box. But it can also generate multiple boxes or no box. I think it should say something like

The list-item keyword creates a ::marker pseudo-element which generates boxes as if it were an immediate child of the element

And consider linking CSS Lists Markers, because CSS Pseudo-Elements Markers seems obsolete.