Impressive, and not a feat I could match. Still, I suspect he make the task much much easier by holding the glass in his hand, and by doing so at a near-horizontal angle. A glass standing free on a table will shake and distort its shape before breaking, which means at near the breaking point the resonant frequency isn’t constant, which means you need more power in your voice to break it. This was a demo my professor did freshman year in a physics class, the distortion was very apparent.
