Gender as a social construct is a new thought, one discussed and accepted by people with an interest in the acceptance of gender difference more nuanced than penis = male, vagina = female.
But most people still think in the obvious and centuries old limited definitions, even if they are, to some extent, theoretically accepting of LGBTQ individually or as a concept that exists beyond their circle. They have not actually encountered the idea that the word gender applies other than as the more discrete and sciency sounding euphemism for describing which body parts you have.
That you think gender as social construct is the standard meaning of the word shows your friends and aquaintances and social media is full of a certain sliver of society. Mumsnet is making you aware of another sliver who do not understand the word in the way you do, because they have not encountered that way of thinking. Or have encountered it, but not yet really understood/accepted it.
I suspect the large portion of the worlds population, who arent on-line nor educated to modern western thought on the definition, also think of the word as describing two physical conditions, rather than a range.