He can still be an amab femboy and take hrt to become more feminine without transitioning to become a woman. He does not claim to be trans, just a femboy.

You misunderstand. Femboys are people who were born with penises.

I've heard from a few transfem lesbian friends that one of the hardest parts of coming out to themselves as trans was that it would also mean acknowledging they're not straight, because if you're someone who feels like you're failing at being a man, attraction to women can be pretty grounding. "It was the one normal thing about me" - whole lot of internalised misogyny/transphobia/homophobia.

But that's our assumption, it's true that it might just be a dude that likes being feminine, no trans involved.

Quite right; labels like straight get pretty wibbly in situations like this. I find it sad because the current prevalence of homophobia and transphobia makes it hard for us to collectively understand what would it mean to be a cis-het femboy who takes HRT — for us to conceptualise of a world where that isn't a contradiction. Bigotry makes it harder for people to explore their true selves, regardless of their gender or sexuality.