does not claim to "slice through glass" at all, just to "score and cut", and indeed that is one of the uses for W-C tools. Tungsten carbide is often used to make the indenter for performing hardness tests on other materials including steel.
For references, the Knoop hardness (measured using a pyramidal diamond indenter) of glass is ~600. Steel at 68Rc is 920, even at 57Rc its knoop hardness is >650. So hardened steel can be used to score glass - but it requires more effort and can result in greater damage to your edge. Tungsten carbide can reach 1800 on the knoop scale, providing for easy scoring and effective cutting when used in a saw blade (basically carving glass).
Keep in mind that HSS at high hardness basically uses carbon to form a carbide-rich martensite edge, and it still wouldn't be as hard as W-Co hardmetal.
As for "slicing" glass, these would be very tiny shavings and chips, it's not like slicing paper and plastic, again think of the chips and fragments that come from running a carbide-tipped saw blade, or the shavings of metal from running a carbide lathe-bit to drill steel. As others indicated, it's not like you could push-cut your W-C knife edge through a 1/4" plate of glass or even 1/8" - the geometry of the blade's wedge-shape isn't capable of splitting apart the already cut glass. Instead, a groove is cut/gouged (i.e. 'scored') until the strength of the glass is over-come and it fractures apart along the channel.
As another point of reference, consider using a knife to push-cut through 0.093" kydex sheet for sheath-making - despite the plastic being MUCH softer then the steel, the geometry of the knife-edge is unlikely to be able to accomplish the task without a LOT of effort - it wouldn't matter WHAT hardness that blade is, it's geometry that cuts and thick geometry means more force required for cutting.
