If someone says their sense of gender identity, which is I think the same or close to the concept of self-image, was of being the opposite sex, that seems quite possible to me. And someone who later sees themselves as trans might well see that as the starting point.
Let me muse... based on my own expereince....
I was a tomboy, was called by the male version of my name for a few years, even as an adult my chosen name is 'non binary'. At 4 and 5 I wanted diggers, cars, transporters and rollerskates. Never liked dolls or dressing up. Had very long hair, wore dresses out of choice. Favourite sports were boxing, played football, to play and watch... in the late 1960s early 70s.
I grew to know I was female, a girl by the time I was about 7. But continued to enjoy more masculine pursuits, always dislike feminised products, girly spink, sparkly shite! Still do. Nobody batted an eyelash. I was me, I was always accepted. Later, in my teens, I would be bullied for wearing glasses, being freckled, being clever and having red hair, but never because I was not feminine enough!
I am hetero, married, female adult.
Had I been born in this century what would I be?