Due to the huge interest in distance running in Japan the Tokyo organisers are insistent a 100km Ultra for men and women will definitely happen. They believe it will add substantial local advertising revenue to help pay for the games.
Apparently they've been trying to keep it quiet as:
"Discussions on the event programme in the existing 28 Olympic sports for the Olympic Games Tokyo 2020 are ongoing, and will be finalised by the IOC Executive Board in mid-2017."
so other sports don't try to undermine them by applying for events in their sports to be added.