I think most blondes who were blondes at least until puberty or high school always think of themselves as naturally blonde, even if their hair darkens a bit with age. I know that's the case for myself and almost every natural (in childhood) blonde I know. I don't really see much of a distinction, since most childhood blondes end up staying blonde one way or another, since they know that shade (usually) works with their coloring. It's still unusual for someone who was a true blonde in childhood to look much more natural as a dark brunette. I know my coloring can't handle a dark brunette shade.
My sister hasn't been blonde since she was about 12, but she will swear on her firstborn's life that she "is blonde". She calls her hair "highlighted" now, not bleached. She'd look fine as a brunette...because that.is.what.she.is.
Whatever gets you through the night is guess...
Why does it bother you if she thinks of herself as blonde, though? What you see as light brown is obviously seen as blonde by others, including the actual hair color charts.
I've never understood the folks who were so invested in calling dirty blondes brunettes, as if they were bursting some sort of bubble. I just don't understand why they'd care.
Sofia and Winona look to be about a 7 on this chart, FWIW:
People just don't count the dirty/dishwater blondes as blonde. Go figure.