I think it's a little easier in the trans case to feel that confidence in your answer? I mean, far as I can tell, the evidence for denial of trans existence consists of "This makes me uncomfortable", "I never heard of this before", "I don't understand this", and excuses.

And yeah, the what-even-is-a-gender question is still up in the air, as far as I'm aware. We know there are people who tell us that they changed their gender - they were one gender before, they are another now - and we know there are people whose gender came years or decades before knowing that gender was real and having language for it, because they tell their stories too. We know there are trans people who hate some of the markers of their assigned gender - being "Sir"ed or "Ma'am"ed in public, visible facial hair or boobs, a low or a high voice, and so on - and trans people who rejoice in some of the markers of their present gender, but the former is definitely not universal and the latter might not be either. We know that trans children answer questions about themselves in ways that match cis children of the same gender. We know that cis children have to develop a gender identity themselves - none of us are born with a gender, as far as we can tell, we have to find out about gender and figure out who that means we are.