Australis said: Digging further, the COL uses KEW's POWO as their source for this genus. Here is the source page:
states the source used to synonmise the two names:
Iwatsuki, K., Boufford, D.E. & Ohba, H. (2016). Flora of Japan IVb: 1-335. Kodansha Ltd., Tokyo.
If you can dig up a copy, that might tell you why they decided to combine the species.