No. If the material is softer, the abrasive will embed in it, forming a lap, grinding the harder material. It doesn't have to be a block of glass, a block of steel will work as well. One advantage of using a block of hard material is the fact that the abrasive will not embed, making it easier to attain a random pattern.
Sorry you're the one who's wrong, if you use a plastic block to grind glass or vice versa ithe grit doesn't embed in the softer plastic. Of course that changes if the material is very soft.