I would think that a low plasticity index or bedrock would provide a higher bearing capacity so the footings could potentially be smaller or not as deep?
You are probably correct. If the footings are designed for a presumptive PI of 32, and the actual site conditions are much better, there may be savings achievable by redesign.
It sounds like it's likely a PT slab due to expansive soil and the PT slab was designed for a PI of 32. If PI is less than 15, it's not considered expansive per the IBC. It sounds like the designer is telling the construction side to reach out to them if the PI is less than 15 or on solid rock as it's then not considered expansive and the PT slab is overdesigned; therefore, the slab can be redesigned and will likely be thinner, less reinforcement, cheaper, etc. Good on the designer.