Well it was her spell, I'd like to think it'd work. Her connection or whatever to the island could be something they could build on later for another retcon. For now they are going with she used her powers in conjunction with that of mutants and that because she believed she was Mags' daughter and a mutant that was enough. I think the reason they had to pull back is probably the Avengers office. And probably to do with the movies. So for now she's just mutant adjacent and Mags considers her his daughter. But I do believe Leah wanted to undo the retcon to the retcon.
With her being one of the most retconned comics characters ever, I have no doubt that in like 5 years that they retcon her completely back into being a mutant again and Mags kid biologically. They'll probably have her go back and forth many times over the next decades.