If they weren't let in, then they have a clear sex discrimination case, following the latest ruling.
If the pool is not using the single-sex exemptions, they can't exclude women.
The could try saying "well, the men's pool is single-sex", but if the women's pool is not single-sex, then that's also sex discrimination.
There is no longer any ambiguity at all here in the definition of sex for the discrimination law.
All we need now is someone with bottomless pockets who can afford the upfront costs of taking everyone to the cleaners.
The only way the pool has of getting out of it is saying "yes, we do use the single-sex exemptions to provide single-sex spaces, and that's why these women were excluded from the male space."