Even though Rakuten is a Japanese company and all this happened in Japan, the department where it happened is almost entirely not Japanese, so the "elevator sales pitch" has nothing to do with Japan. There were already several developers that were on board with Functional Programming, this is what helped a lot.
As a matter of Japan, there is a very lively Elm community and I know few companies that use Elm in production.
Also, I heard that the Japanese academic world is active in Functional Programming research