At that scale the meaning of "surface" becomes a little weird. Hell, at the nanoscale lots of things become weird. "Glass is hard but brittle" doesn't necessarily apply.

The article says the coating it flexible. If the silica molecules are bonded more to the surface than to each other, it may be that you'll scrape a larger (say, micrometers: 1000s of nanometers) bit of the object away, glass coating and all.