We need to know if this was a Magritte-style swimsuit. Something with a Schrödinger-style (non) declarative statement about being non-binary. Expressed, of course, in plain enough English, perhaps with a diagram, so that every tiny child or other pool user would instantly, and without ambivalence, have understood this celebration of non-binary personhood that was taking place in the lane of a local swimming pool attached to a sports centre in Scotland.
However, even in the context of the above, I wonder why the Reddit 'concerned relative' is so quick to dismiss the content of the woman's complaint? Have they, perhaps, considered asking the relative in question to change into the swimsuit and have a quick stand under the shower to simulate the conditions in question?