Seems like they ramped up Luci-chan power in the manga? MC did say she was powerful in the novel, but I don't recall demon lord or god level from the isekai, and she isn't even the strongest of her family, just really strong.
MC too is limited, he can't go all out as his body is too young, to be exact he can do short controleld bursts but even using the hero sword pushes him too much.
I remember that in the novel it was a short fight that ended with him bisecting her, in the manga he exploded her waist area...close enough I guess.
And I wonder if the chapter was partially MTL and not checked as MC refers to she as a "he" 3 times in a row.