I came here looking to suggest that was their angle, so I'm glad you called it.

Was more progressive in that single sense, probably (also at least some pre-Christian Europeans like Greeks and the Norse had a different paradigm where it was fine to be a male top but not a male bottom [atimia, nithling respectively], but that's sexually and not gender). Anyone who says it still is like that is an idiot. As for why Africa was but no longer is decent on accepting gays and transgenders, some SJW might argue the spread of Abrahamic religions into such areas via colonialism spoiled the broth so to speak (if the SJW didn't just cop out and blame white people). And, stripping away the noble savage myth they apply to precolonial anywhere, they wouldn't be completely wrong. Muslims and Christians have historically not been kind to anything but cis people, and it's only recently that Christians in secular countries are more tolerant.

Also the idea that third gender or two-spirit people were accepted in precolonial Africa and the Americas comes from several observations from various missionaries that men who wore dresses weren't killed or ostracized by whatever tribe they lived with. A), that's not a super high standard of progressiveness unless speaking relatively, and B) there was probably some selection bias, like maybe the missionary wouldn't have bothered to record another gay person being mistreated if he were in a less tolerant tribe.