I had the same thought, but actually the super long timeline could also be evidence that they're taking it seriously. Roppongi Hills, another Tokyo development, took about 18 years from the start of planning to the end of construction.
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)
Given the Roppongi Hills example, provisioning 15 years just to acquire the smaller properties seems reasonable (and completely crazy at the same time, depending on the point of view).