Can't dispute your detailed explanation of airfoil
terminology but it is very common to designate an airfoil that has a concavity to it in the bottom surface as having "undercamber". Perhaps this is "modelese" but even Andy Lennon on page 10 of his treatise on R/C Model Design, refers to the bottom airfoil curvature line as the bottom camber line and further, "the reverse curvature of the lower camber line as being an airfoil with undercamber".
So, I will continue to call my model wings with concave lower camber lines as undercambered airfoils and the GWS Slow Stick sure has that property.