Yes, it seems a bit disingenuous to say you have to provide separate toilets but hey, who knows what happens after that?
It seems obvious that if an employer correctly provides separate toilets, correctly signed as being exclusively for women/men, they should expect their employees to use the facilities accordingly.
They can't police who uses which, but they can and should clearly state that it is company policy that single sex toilets are to be used according to biological sex, not according to individual choice.
A male employee who persists in using the women's toilet would be refusing to comply with a clearly-stated policy. An employee who persisted in parking on a space which was reserved for deliveries would be told to stop, and would face some kind of sanction if they refused to stop, so the same should apply to the man using the women's facilities.
I think all that was expected of employers in the wake of the SC judgement was to assert that they were following the law and their single-sex toilets were separated on the basis of biological sex. Nobody expected them to have a security guard with a swab kit at the door, but it wasn't too much to ask them to assert the rules, and be open to dealing with complaints about persistent cases of breaking the rules.
Not the free-for-all that this judge seems to think is OK.