Well, the hygene could be reflected upon the pool’s water chemistry.
Wearing ‘full clothing’ not only brings in volumes more of detergent residue (phospates); but pockets can carry all kinds of things damaging to a pools skimmer/return lines or vinyl liner (if not concrete). That, plus ‘flowing’ garments could lead to safety issues even with drain/suction updates (do they even have those outside the US?)
The Paris pool manager is quite correct.
The floss that passes for bathing suits at the beaches and pools these days bring their own set of hazards. Whiplash for instance and the resulting injury when my wife returns my head in her direction.