At quoted "1.5 GB/inch^2" they are comparing their 100 layer medium against a single-layer 25GB bluray disc. At three~four layers, current bluray discs offer 100~128GB of capacity.

Archival Disc spec brings optical media to 300GB now, 1TB on roadmap.[0] This leaves current Quartz Glass an order of magnitude behind current optical media.

[0] https://en.wikipedia.org/wiki/Archival_Disc

*fairly updated source

Article was 2012, the press release was 2014