The systray container plasmoid is not a containment, but is a normal applet that does some hack to display the contents of the internal systray 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 systray is listed in the "add panel" menu, however it doesn't really work there. that key was wrong from there to begin wwith. After some testing there doesn't seem to be any behavior difference of the systary plasmoid when used