For me, when you explain that being trans has nothing to do with gender expression it leaves me feeling totally clueless about what trans does mean. That's a bit difficult for me, the fact that I just don't get it, but doesn't make it your job to satisfy my curiosity and explain things to me so that i can feel like I do fully understand.
I would like to know though, does it matter to you either way whether I (cis person) understand what it means to be trans - or does it actually only matter that I respect you as a person and treat you how you wish to be treated ?
I'm not the poster you quoted but I can give another trans person's perspective on this if you're interested.
I'm not a ... whatever kind of scientist looks at this stuff (?) I'm not one of them so I don't know if this is right, but something that made a lot of sense to me when I read it in a discussion was the idea that gender ROLES etc are a social construct, but having some kind of internal gender identity is inate. (I think that was touched on earlier in the thread, a suggestion that there's a genetic element?).
As a side note, I do wonder whether it's even possible to get rid of gender roles etc completely or whether the best we can hope for is stereotypes that are much less harmful, not linked to oppression etc and not enforced.
So anyway, you have your internal gender identity which is who you are; gender roles which is what society says [gender] should do; and gender expression which is how you express your identity through manipulation of the roles available. They're not entirely separate things, but gender expression and identity aren't linked in a simple, x=y way.
To give a personal example, I didn't choose to transition so I can lift weights and do martial arts and have short hair. I could, and did, do all of those things as a (often gnc) woman. Being masculine or feminine wasn't the problem - being a woman was the problem, for me. I chose to transition because my body on testosterone is so, so right and wonderful and having breasts is just wrong. I chose to transition because as far back as age 4 I remember looking at the boys, or later men, and having a feeling that I should be there. Not because of anything they were doing, just a certainty that
I should be in that group too.
There's a thousand little things I could list and none of them are specifically what determines my gender, but they all add up to a bigger picture.
Some of the "masculine" stuff I do is unconscious; I just do it without thinking and always have done. Some is a choice but unrelated to gender expression; I do it because I want to, not because it makes me male or female. Some is a choice and related to gender expression; I do it because it's easier, or I don't want to draw attention to myself at that moment, or to avoid being read as female.
Same with "feminine" things. Some are unconscious; I just do them without thinking. Some are intentional; I do them because it's who I am or what I want to do. Some I do intentionally because fuck nonsense gender bullshit.
Cis people do the same thing. We can all choose how and to what extent we use gender expression & roles/stereotypes to express our gender. They only express gender though, not determine it.