The reason why Roppongi Hills took 18 years was because much of that time was spent acquiring lots from 400 smaller property owners. The construction of Mori tower itself took ~3 years.

This proposal does seem more marketing like marketing than a serious attempt to construct a building in that neither the website nor press release mentioning the location that the tower will be built (Marunouchi).(http://sfc.jp/information/news/2018/2018-02-08.html) (http://sfc.jp/english/news/pdf/20180214_e_01.pdf)