They haven't evolved anything. Please read the court's opinion, the logic they use to describe why "sex" from Title VII isn't divisible ("but for" test) from LGBT rights is extremely difficult to refute.
Nothing has "evolved." They're arguing that you cannot descriminate against LGBT people without ALSO descriminating against someone's sex, which is unlawful. Even the dissenters weren't really able to poke holes in that logic, instead ignoring the whole issue of how someone's sex determines if they'd get decriminated against in some circumstances and not others (e.g. employers banning being married to a women is only "banned" for one gender and not the other, making it sexual descrimination).
They give numerous examples and go into great detail about why it is just sexual descrimination with extra steps, exactly like Title VII was expressly designed to outlaw.