There's a useful observation in Simone Weils Gravity and Grace, edited Gustave Thibon from her notebooks, which might help your confusion on this:
Literature and morality: Imaginary evil is romantic and varied; real evil is gloomy, monotonous, barren and boring. Imaginary good is boring; real good is always new, marvelous, intoxicating. Therefore 'imaginative literature' is either boring or immoral (or a mixture of both). It only escapes from this alternative if in some way it passes over to the side of reality through the power of art - and only genius can do that.
(Gustave Thibon classified this observation under the category of evil, in his thematic division of her book).
Given that she thought Shakespeare generally a second-rate author of plays (apart from King Lear), her standards were very high (but of course ought to argue that standards ought to be high - otherwise why have them?) So, I don't think she would count Milton as the kind of true genius who had the power of transmuting imaginative literature into the kind of transcendently real art, the kind of art that points at the real, even on the unreal plane that imaginative literature plays on, that she was looking for.