If you listen just to the voice, not quite prog. Listen more carefully to the musical compositions in the background. And then remember a lot of that was mid to late 70's and early 80's. That was very progressive.
Her style was unconventional. That also made it more progressive.
It really, in most circles it is considered "Art Rock" along with Brian Ferry and Roxy Music, and that gets lumped intogether with prog these days.
But that's just this ole hippies opinion.
And besides - she's just a babe. Not many of those in the prog scene. Need more!