Not because I don't think it's true, but the Koreans have done similar 'research' in the past claiming that things in China and Japan originated from the Korean Peninsula. Japan became a notable civilization long before the Korea did, so it's hard to even say that the "Japanese came from the Koreans". Back in the day, there was no significant nation known as Korea. The central focus was on the Chinese Empire, and Nippon (Japanese Empire). Korea was colonized by smaller kingdoms that were of little to no significance in terms of cultural and political impact. This made Korea like a bridge between China and Japan, as it was easy to navigate across the sea through the Korean Peninsula. Cultural exchanges between the Chinese and Japanese eventually became the traditions of the Koreans as well, and it is shown today with the many traditional holidays in Korea that did originate from the two other countries.
There are many borrowed words in the Korean language that originate in the Chinese and Japanese languages as well, and given the dates in history, it's hard to believe that the Japanese language would actually originate in Korea. Based on cultural regional powers, I think it'd be more believable in vice versa to this research report. The Chinese, being the largest power, has a bigger cultural impact, spread their written language over to the Japanese. The Japanese, being a notable civilization longer than Korea, developed their regional language, and likely through trade and interactions, had their language come back westward to the Asian mainland. Korea, being sandwiched in the middle, developed their dialects from their own ancestor's dialects plus the influence of their powerful neighbours.