You don't know that. And I don't actually know that it is. I think it definitely has the characteristics of glass though. It's reflecting a light source, which is why it doesn't look exactly the same as the Elven Glass in the Wraith picture.
Glass is a material. Claymores are of Breton origin, yet there are Glass Claymores.
As for the Scimitar itself, what province uses Scimtars? Yah, that's right, the Dunmer of Morrowind.
Actually... the Khajiit of Elsweyr use Scimitars more. And Redguards of Hammerfell.
And Dwarven stuff is common because while it's true the dwarves disappeared millenia ago... they left
lots of Dwarven stuff behind, and it doesn't stay broken.
I liked Oblivion's glass armor: The lightweight glass plates looked effective at deflecting indirect blows (Which glass's hardness and brittleness makes it particularly good at), while the metal wiring gives it the flexibility to bend under blunt trauma, as well as move freely over the wearer's body.