I think though that they are trying to get round it by saying that mixed sex “single gender” spaces pretending to be single sex aren’t actually illegal.
Mixed sex spaces pretending to be single sex are illegal. They just need to get the signage right and appropriate to the facility.
In the workplace, single sex facilities must be provided if needed as part of the working environment (such as the need to change clothes). They can't 'get around' the law by claiming that gender and sex mean the same thing.