The issue is not always the small area of the pool itself, but all of the connections with the pool at the facility. The mistake people make is remaining in the pool area with lightning nearby. Leaving the water is not the end of the storm, but the beginning of lightning safety.
A flash can hit a power or phone pole outside the pool, or it can hit part of the structure itself. A cloud-to-ground flash to the pool complex can then travel easily through standing water, plumbing, wiring, fences, and phones to affect many, most, or all people at the facility.
the National Servere Storms Laboratory online.