This is an amusing term used by those who favour nude beaches for a beach where people wear swimming costumes. You can also refer to a person who wears a swimming costume as a textile.
For the majority of people who go to the beach, the nude beach is the exception to the rule, and therefore has a name, whereas the beach where people wear clothes is the norm and therefore nameless. It is just the beach. But for those who favour the nude beach, that other kind of beach needs a name as well. Textile beach is pleasantly light-hearted. And of course nudists and textiles then becomes the related pair.
The term nude beach is seen as carrying negative connotations so there have been attempts to replace it with free beach in the sense that the people who go there are free of clothing. The more intuitive understanding of free beach would be one where you don’t need to pay money, so I think that nude beach or nudist beach will probably continue to have currency.