How I broke a wine glass with my VOICE (using science!)

That's actually why fiberglass is so strong. They make glass fibers which are absolutely perfect and then immediately coat them to prevent them getting any damage whatsoever. Then when they encase the woven fibres in a polymer matrix it strengthens it, particularly under tension (but also somewhat under compression- the fibers are super strong under compression, provided they don't buckle, and the matrix stops that.) But a wine glass that has never, ever been touched should be as strong. It may well be that the glass you broke, broke where it did, because that's where you had been handling it the most and you'd created microscratches there.