https://www.demetrialucas.com/blog/62o3erzgnxfn2wbwx6o1cedlbump0z
"Example of stupid sh–: Biltmore has an indoor 70k gallon swimming pool, but technology of the time didn’t allow for a proper cleaning system. So they know how to fill the pool, heat the pool, and light the pool, but anyone who took eigth grade biology should know what happens to sitting water, especially in a humid environment after a couple days. (Short answer: it becomes a science experiment). The other problem: swimming wasn’t popular. Most people didn’t know how. So yes, there’s an indoor pool, and that’s really impressive, but you can barely use it."
In other words, they didn't use it as a swimming pool.