You've obviously rushed off to check what 'legal fiction' means, having waved it away earlier as some kind of madey-uppy GC nonsense, and realised that it's an established legal concept.
You even saw that the classic example of it is adoption, where a 'legal fiction' is established: that the adoptive parents are the parents of the adopted child.
That's one of the things that characterises some posters - they are so often wrong. Not in a moral or ethical sense, but factually wrong. Like saying that they assume that 'legal fiction' is part of the 'lingo' of sex realists.
And then when they google it or whatever, to act like of course they always knew it
Or saying things like they are a 'woman legally' although it has been established that said a biological male remains legally a man.
Or 'sex realism' aka provable scientific fact, is a 'niche obsession'.
I don't know lots of stuff, I get things wrong, but I make a point of checking my facts before entering into a discussion. I don't want to look like an eejit by saying clearly inaccurate things like 'legal fiction is a phrase recently made up by sex realists' or 'sex realism is a niche obsession'.
It interests me that people like onepostwonder don't seem to mind showing time and time again that they are eejits who don't know lots of stuff and get things wrong.
And then they sail on unperturbed by having demonstrated, in writing, how ill-informed and mistaken they are.