Swift J was being thorough: since a 'trans-inclusive' service lies outside the Schedule 3 SSEs, it must be examined for sex-discrimination on its own merits, by doing a 'less favourable treatment' analysis.
He sufficiently agreed that there was a potential liability, arising from the different access criteria for the two sexes, to justify including a statement to this effect in the Interim Update. He couldn't rule that there would always be illegal discrimination, because that might depend on the facts of the case.
For example one could argue that the Ponds, taken as a whole, are distinct from Al Hijra in that each user has the choice of two different types of mixed-sex service, rather than being segregated from the other sex.
Intuitively, trans-inclusive spaces should be unobjectionable for people who want them. The problem exists for people who dont want them, and for whom market forces have failed to provide an alternative.
Depending on the evidence, mixed-sex services can be illegal because they are more disadvantageous for women than for men. But outside of specific regulations they do nevertheless exist because it suits providers and (in the form of 'trans-inclusion') is pushed by activists. We can't sue them all, but we can lobby for single-sex services to be mandatory in a wider range of scenarios. This might not stretch to recreational swimming.
And, we should make more consumer choice type noise about incorrect space-labelling and the fact that regulations around mixed-sex toileting lead to safety and hygiene issues.
I wish that the Jewish people of North London would sue, because they've been unable to use the Ponds since they went mixed.