They weren't talking about using the manufactured blocks to make the whole thing, just the top portion of the stones used as a dressing, which would be a small percentage of the total blocks used.
The dressing could not be soft cast chalk- with the blowing sand from the desert next door it would be eroded away in a nick of time. If anything, the pyramid exterior needed to have been made of the hardest materials available, for the whole point of having a pyramid is its extreme durability. To cast the pyramid interior [the bulk of its volume] would take a lot. And allowing the soft castings to settle and harden to the depth of the pyramid dimension would take longer than the whole reign.