Cisnormativity assumes that a whole host of characteristics can be neatly divided into two distinct genders, modulo whatever infinitesimal exceptions they’ve been convinced to make. IMHO the sex/gender axis is only one way to subdivide that mess of assumptions. Despite its emphasis in gender studies, I’m not sure it’s the most meaningful axis. After all, it’s difficult to sort a whole host of gendered characteristics into distinct biological or cultural boxes, especially in a world where culture shapes biology, in the form of workout regimens, body-shaping clothing, surgeries, and exogenous hormones – all used by both trans and cis people to conform to cultural ideals.
Aside and pure speculation: the sex/gender distinction almost seems like an accident of the English language. The language obtained these near-synonyms, and so it seems natural to interpret them along English’s traditional germanic/latin distinction. “Sex” isn’t germanic in origin, but it sounds like it is, which is maybe good enough 
sex : gender :: pig : pork
Sex is to nature or “the raw” as gender is to culture or “the cooked.”
– Judith Butler abridged by Riki Wilchins.
In any case, I think their conflation of sex/gender here is only a symptom of conflating all gender axes, including presentation, hormonal differences, birth sex assignment, chromosomes, etc etc. I doubt there’s any subtlety intended by the bus’s designers 
