Yes that's the SC ruling. A single sex space/service/requirement is single sex, based on birth sex. Reassigned gender does not replace or override sex.
If a transwoman applies for an ordinary job and is refused for being trans, that's illegal discrimination on the basis of having the protected characteristic of gender reassignment.
If a transwoman applies for an ordinary job and is refused because the employer thinks she's a woman (and the employer is a sexist who thinks women are no good at whatever the job is), that's illegal discrimination on the basis of being perceived to have the protected characteristic of (female) sex.
If a transwoman applies for a job that has a genuine occupational requirement for the holder to be a woman (such as, to pick a totally random example, the head of a rape crisis centre), then a refusal on the basis of sex is legal.
If a changing room, support group or whatever is single sex, then by defintion it excludes trans people who are not that sex. A women's service excludes transwomen, a men's one excludes transmen.