slurry backfill or base is a cement and sand based stabilizer, similiar to concrete except much less cement content. It is commonly used in Texas given your soil conditions. Most if not all your highways have a slurry base under them. Given the torrential rains you can get and soil properties it helps keep the base from washing away. I can't tell you if its appropriate for a pool for base, (can't see why not) but we use it all the time in the construction projects we do in Texas. Also limestone in Texas depending where you are can be very porous with clay seams, this will get crumbly over time with rain and the heat.
I just saw that you were in Waco, the limestone in this area is very spotting and not very good quality, it has a lot of clay seams in it, this propably explains why they want a stabil base. Though I'm not a PB.