Considering Tia and Ru got their asses wiped by the spiders and wolves, and they're super strong, I won't get surprised if both species are demon lord level
Considering that it doesn't exist in that world and he can recall anything that happened in his life, and as the system said that he did a better path than the original author (implying that he changed something?), maybe no. It isn't plagiarism if it doesn't exists.👻