Hard as stone is probably meant in a figurative sense, not a literal sense. End-grain wood is pretty hard, especially when compressed (for instance: by beating it with a hammer), but it is nowhere near as hard as stone. That said, there are some pretty soft stones :)

I've broken more than one high quality steel bit on various kinds of wood, but that's usually not a measure of how hard the wood is but how much friction there is between the drill and the wood and how much of the drill bit is inside the wood. If the wood were really hard you would not be able to enter the bit at all and it would never break but just sit there spinning on the surface.

Finally, the whole trick to a cutting board is that end grain wood will wear very little and the knife will be sharp much longer specifically because it is not as hard as stone.