The answer is simple: it provides maximum performance. There’s a reason why you won’t see a single diver wear swim trunks or board shorts. Sure, those are way more modest if that’s something they cared about, but they would be shooting themselves in the foot competitively.

The first reason is that freedivers need to minimize their clothing to reduce drag and resistance while swimming. The second reason is that freedivers need to avoid entanglement in their clothing while diving. Wearing a thong helps to accomplish both of these objectives.