I have not read every side of that bus, but how it seems to me is that they are explicitly stating that sex is biological, but passive-aggressively implying that sex == gender so that gender is biological also. Its XX and XY also depict one tightly constrained marker of biological sex, reenforcing the notion that biology means a binary rather than a continuum. It seems calculated to function as a dogwhistle for essentialists, while saying very little outright.
An amusing contradiction I would press them on is that if it’s all simple biology, then why are the “boy” and “girl” glyphs visually reduced to one of them having a dress and pigtails? Does that mean they are fine with boys wearing a dress and having pigtails, just so long as they know what chromosomes they have?
