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I think the teams involved does play a huge role here. Nunez is a very talented player as well and could very well be relied upon to score goals. Manchester City plays beautiful football as a team and does a pretty good job of getting the ball over to Haaland who then goes ahead to score goals.
I agree with you that if the roles were reversed and Darwin Nunez was playing for Manchester City, we would see Nunez score more goals and Haaland score less. I think the entire team hugely contributed to a very successful first premier league season for Haaland.
Liverpool and Manchester City are two different teams. Klopp and Pep Guardiola have different game philosophies that affect the way the team plays both as a team and individually.
Haaland is a target man at Manchester City, while Nunez may not have that role. Darwin Núñez can play in two different positions and he can play not only as a center striker but also as a left winger. Nunez even has good statistics when playing on the wing, he has 5 goals and 2 assists in 10 appearances. But Nunez struggled to find many goals when he played as a center forward, his statistics were 4 goals and 1 assist in 19 matches.