Bit of a misrepresentation in the OP (which is becoming a trend from you
@alanb).
The ruling was about the duty to disclose. First, they didn't say that trans-women aren't women. They were reusing the specific definition from a lower level hearing.
This was an appeal from summary judgment. For the non-lawyers, it basically means that the plaintiff didn't state anything in their complaint for which there is a violation and the defendant wins without having to try the case.
Here, the woman is saying that she did state something that violates the law - specifically that Planet Fitness misrepresented who has access to the bathrooms. And that she might not have purchased the gym membership if she had known.
The court ruled that Planet Fitness has a responsibility to tell people that they (PF) use self-identification for access to their locker rooms. PF says it is their policy but it's not written in the contract. The case was sent back to the lower court for trial consistent with the position that PF has a responsibility to disclose this information. So judgment without the trial was inappropriate.
They made no ruling whatsoever on if trans women are women, if trans people should have access to the bathrooms they self-identify with, etc. I'm kinda disappointed that
@alanb who seems like a good lawyer would post this misrepresentation of the ruling.