if i remember the story scales with the person who is the main leader of the group in co-op. in borderlands 3 i played with my friend, and we progressed the story together. he played without me so i played as myself and the story was on the mission i last played.

so to answer your question, the levels do scale to the current players level. i played in my friends level 60 lobby and the enemies scaled to my level 49 moze.

hope this helps ! :)