They have used the screed rails and laser...and here is the gravel they are using.That pool is a flat bottom. They should add small gravel and screed the base after compacting. On a small pool like yours I would use screed rails shot with a laser to the proper pitch. Gravel should be used to bring backfill near the step bump in then masonry grout to stuff it to the top. Then continue with water and stone and the same process for the whole shelf. A delivery of flowable fill would be the best bet and guarantee no voids. Make sure the gravel used is 1/4 to 3/8 max size and angular rock not rounded gravels