There is a building which is clearly visible on aerial imagery.  The history of how it was added looks normal (the user seems to have consciously added toilets here), and it is inside a fenced camping site.

For me, this looks clear: this is a small building with bathrooms for users of the camping site.  It is correctly tagged as access=customers because it is not supposed to be open to the general public.

I don’t understand why StreetComplete is asking something here…