[1.1.0] nav_menu component : extra content in the case of mega menu

Problem/Motivation

in the twig file of nav_menu component, there is reference to things like

parent.content
parent.content.extra_title
parent.content.extra_description
parent.content.extra_link
parent.content.extra_link.url
parent.content.extra_link.label

where parent is a link from links prop type.
it seems this is code converted from ui patterns version 1 to version 2,
but to my understanding, parent.content does not exist.

we need to clarify this point and this component.