The 'line' is that it is very complicated. There are people with strong body dysphorias who have always had them, there are people who care much more about social presentation than anatomy, and everybody in between or in combination. The social presentation can be separate from body image considerations, and for those people in particular 'wrong body' would be inaccurate. There are people whose experienced-from-within gender is different at different times or who do not strongly identify with masculine or feminine, or with bits of both. Knowing multiple people in various positions on these spectra, trying to collapse the experience of everyone with non-default gender situations to one party line is a recipe for confusion, unproductive arguments, and missing the point for some of them.
So was Jenner always a woman trapped in a man's body? The answer according to his (her) transition miniseries is yes (complete with tales of sneaking into his mother's closet to wear her clothes).
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.