I think when spectrumers say they couldn't imagine at all what they'd be like if they weren't autistic, (if asked most do typically say "I don't know") it's because they probably are quite severe, or come from a family with more NDs than NTs, or lack imagination. But there are a few of us that can imagine what we'd be like if we weren't on the spectrum, but still in the same body, from the same family, etc.
I guess it's like a girl imagining what she'd be like if she had been a boy, and vice versa. Obviously it's not accurate but those of us with imaginations can take a guess and get an idea of what we'd be like.
If a girl grew up in a family full of boys, maybe two (gay) dads, many brothers, and not much contact with female family members, she might begin to wish she had been born a boy like the rest of her family she knows of, and might imagine what her life might have been like had she been a boy, being so she's been around males all her life (maybe homeschooled?) and so been observing nothing but males. It can have that effect.
So it's a bit like me with ASD/ADHD. I've always been the only one diagnosed with those two conditions in my family, and in my class at school, so really I grew up surrounded by NTs and so have had no choice but to observe NT behaviour and put myself in their shoes. In fact I understand NT behaviour more than I do autistic behaviour.
To use an even better analogy, a person who grew up in the jungle might identify more with animals than with other humans and might wish they were an animal and so feel uncomfortable in their own skin, and have been around animals enough to observe their behaviour and want to become one.
That's exactly my experience with AS.
I have one cousin who I feel has a lot of spectrum traits even from a child, but he was never diagnosed, and every time he posts photos of himself on Facebook it's always been with friends at bars, clubs, pubs, music gigs, stag dos, new years eve parties, etc etc. Is it possible for an Aspie to have such an active social life like that?
So whether he is on the spectrum or not, I still feel alone, because I got diagnosed so early and I've always been the stereotypical shut-in.