It’s not a myth, he’ll score more than what DCL did last season if he played as much.
DCL has a lot more in that he can hold up play (something he’s improved massively), is great in the air and plays some nice through balls (another attribute he has improved on). However, Niasse is a better goal scorer.
Does that mean that we would be better with Niasse as first choice? Absolutely not. As funny as that would be and as much as I like Niasse for his effort, he isn’t good enough; but neither is DCL (not as a first choice striker anyway).