To be clear (for the benefit of anyone else who reads this), the windows in the Seattle Tower are made of glass, but the exterior of the building is not that modern all-glass-and-steel look[1]. This is a third interpretation of non-glass I hadn't thought of and it took me a minute to figure it out.

"non-glass skyscraper":

1. No glass used in the exterior construction at all -> implying no windows

2. No glass used in the exterior construction at all -> implying the windows are made out of something other than glass

3. A skyscraper in which glass is not a prominent architectural feature, but the building does contain features like windows and doors that contain glass. (This comment)

[1] https://en.wikipedia.org/wiki/Wilshire_Grand_Center