given that (in physics) sound is vibration, I think a clearer way of phrasing it is that glass cracks at the speed at which a vibration can travel through it. glass is much denser than air, so the particles can knock into each other more easily, meaning that the speed of sound (or vibration) in glass is a lot faster

(this isn’t aimed at explaining it to you, just to write out my thoughts, as I found their explanation a little odd)