That's my reading of it, spannasaurus. It's about discrimination based on the 2 protected characteristics intersecting.
You could have a group for men with the protected characteristic of gender reassignment.
You could have a group for women.
But if you have a group for women and men with PCoGR, what is your legal basis for excluding men without PCoGR? You can't say 'because they're men', because you let some men in. And you can't say 'because they don't have the characteristic of gender reassignment' because you let in women who don't have that. And what is your legitimate aim?
It's like the current Girl Guides rules. All girls allowed, some boys allowed, other boys excluded. It's not been legally tested, but I think even before this ruling it wasn't actually within the law because it's an incoherent grouping with no legitimate aim.
Redheadedness isn't a protected characteristic, so there no prohibition on discriminating against someone on that basis. Although one could argue indirect discrimination on race.