Notably, this vid says that Geunchogo hires mercenaries from Yamato Japan to help put down the Mahan.
I don't believe that happens in the show.
This massacre also isn't mentioned in Samguk Sagi either, but this story appeared in Nihon Shoki, which is no doubt the source used for this video. In the Nihon Shoki though, the passage says that "they moved their troops, turning westward and arrived at Kohaejin, where they slaughtered the southern barbarians of Ch’immidarye and granted [their territory] to Paekche. Their king Ch’ogo and Prince Kwisu, also commanding troops, arrived to meet them."
Even if the usage of Yamato soldiers to conquer the southwest regions was true, if the show adapted that text, it probably would have invited ALOT of backlash. Japanese soldiers invited to kill native Koreans protecting their home, and the Korean who allowed it happen being painted as the good guy.