My armchair theory after reading this a while back as well was gen Z, better than any other group, understands that gender/sex/etc. is a spectrum. So if you ask them “are you heterosexual?” it’s no surprise that more are comfortable saying - and have the requisite knowledge to say - something along the lines of, “well I wouldn’t say I’m straight because while I am man with a woman I could see myself with a man.”

I don’t have a source for this of course. I’m operating under the assumption that gen z has more internalized the idea of gender/sex/sexuality as a spectrum.