The grouping container plasmoid is not a containment, but is a normal applet that does some hack to display the contents of the internal grouping applet which is one.
Since plasma-framework is built with if KCOREADDONS_BUILD_DEPRECATED_SINCE(5, 89)
now everything that has the containmentype key is considered as a containment, and now the grouping plasmoid is listed in the "add panel" menu, however it doesn't really work there. that key was wrong from there to begin with.