Yeah, it looks like you’re right. Not sure what I read years ago. This is what Wikipedia says:

Mikao Usui originated the practice in Japan. According to the inscription on his memorial stone, Usui taught his system of reiki to more than 2,000 people during his lifetime. While teaching reiki in Fukuyama, Usui suffered a stroke and died on 9 March 1926.

So, apparently before 1926. Still, really far away from “ancient”.