I have read that article before and I think they were all bisexual from the start. Not all female people who transition are into into men after transition so I don't think testosterone creates attraction to men.

The first one, Kissandra says before going on testostoerne "I was primarily attracted to women", so she was bi, according to her own words, as she wasn't exclusively into women.

Emmett says before going on testosterone "maybe I had a couple of crushes on guys, but it was never anything substantial”, so she seemed to have had indications her bisexuality before too.

Tucker says "I was very uncomfortable sexually until I started testosterone. I definitely would never have sex with men before testosterone, but I've had sex with plenty of men after. When you have sex with a man as a woman, there's definitely the dynamic where you feel like a woman". So Tucker seems to have avoided men sexually pre-transition not due to lack of attraction but due to not liking the heterosexual dynamic.

I think Jake was a late bloomer bisexual, testosterone might increase the sex drive so taking testosterone make latent attractions more noticeable.