Because this ISN'T what the judgment said. My bolding of the last sentence.
"61. Whether different treatment is also less favourable treatment is, therefore, a qualitative question. In a case where the provision of separate lavatories labelled male and female was materially similar in terms of the extent of the provision, location, and so on, I consider there would, in principle, be scope for a strong argument that a rule or practice that permitted trans women to use the “female” lavatory but required other biological men to use the male lavatory would comprise different but not less favourable treatment on grounds of sex. However, the circumstances of the case would be decisive.
(For the purposes of the EA 2010 the lavatory would be mixed-sex, but for the purposes of the Claimants’ submission in this case it would still be labelled “women”.)"
What the Judge is saying is:
Excluding other men, but not trans women, from a women-only space, (e.g. women's toilets or the Women's Institute) is NOT necessarily unfair treatment for the 'other men'. Providing facilities of equal 'value', are readily available.
(I'd argue but what if I'm a teeny tiny gay pacifist who has experienced male violence. Those other places still have the scary men in them. Why does the TW get a pass and not this man? So the 'not necessarily' just means it's not a slam dunk of an argument.)
All the other arguments, including just one trans women being there turns it from being women-only into a mixed-sex space, remain valid. One tiny peanut makes a vat of chocolate no longer nut-free.
Hence the bolded sentence from the judgment. And hence why all men, including trans women, must stay out of all women-only spaces.