That is incorrect. I have been to the Nikolai-do Cathedral in Tokyo several times over the last couple of years and each time there were at least two Russian priests or bishops present. I personally spoke with them and they also sing parts of the Liturgy in Church Slavonic from time to time, as does the choir. The metropolitan is Japanese, and I have never seen deacons and subdeacons who were not. The clergy is impressive in their command of the English and Russian, making confession easy for all parishioners.
I was particularly impressed with the knowledge of the Japanese language of one Russian priest, who gave homily in Japanese last May. His mastery of Japanese is indeed impressive: he has been in Japan only 12 years. So, your source is incorrect.
Thank you for your clarification. Perhaps what was meant by "are now of Japanese origin" was that new clerics are Japanese. Thanks for the input.