I think he meant in the Section/Elevation windows, specifically.
In which case, yes you can edit (just) the slab thickness directly since that is the only attribute that applies in the Section and Elevation window that allow for editing only in one plane (vertically).
And it also makes sense why you can't edit it otherwise and in other ways - (as in, the nodes to change the shape of the slab, since the shape is something that's created and defined in plan view (or 3D view) in a plane that's not accessible in a Section or Elevation window and with axes (front and back) that don't apply in a 2D editing plane such as the Section and Elevation windows are.
You can "drag" a slab (i.e. move it) in a Section or elevation window as well as drag a copy..
But I don't believe that's the issue being raised here.
Walls (and to some extent, roofs) are different in that if te reference line is parallel to the viewing plane of the Section or Elevation window, then it will allow you to "Stretch" it, parallel to that plane, but you likewise can't edit them in that same respect of the reference line is not parallel to your viewing plane or worse still, perpendicular to it.
You can otherwise always stretch the height (same logic as adjusting the slab's or roof's height or thickness), and with custom profile walls, you can also "stretch" them via their defined profile stretch lines.
So I guess the short answer to his question is yes you can "drag" slabs in Section windows (if by "drag" you mean "move") but no, you can't edit them (via its nodes), - specifically the defining polygonal reference plane to change its shape in Section or Elevation because that counteracts the logic of working in Section/Elevation.