That’s surprises me, but I'm not near my car at the moment, you tried sticking a magnet on yours, or what makes you say it is ali? If it is indeed aluminium then it most definitely won't be weighing 17kg that is for sure!
Geoff steel seem to think it's steel too, certainly the m3 one I removed was.
https://www.geoffsteel.co.uk/shop/bmw-e86-z4-roof-skin/