Firstly, this "this "gender is a social construct" is a bit dumb" is a bit dumb. You might disagree with the statement but calling it dumb automatically invalidates it and the person who stated it. Please be more careful how you phrase things.
Secondly, just because biology has found different brain structures in different genders doesn't mean gender isn't a social construct. These differences in brain structure simply result in different characteristics and society made the connections between certain characteristics and gender identities. It started out as an identification of typical characteristics with different sexes and when some people didn't fit in with the norm and didn't want to hide it anymore the concepts of gender and sex split. But gender is still a societal construct to group people together by characteristics. And gender dysphoria stems from having been influenced by that system to the degree that they feel like their characteristics should be in a different body.
I'm absolutely not saying gender dysphoria is not valid and that people shouldn't change what they don't like about themselves because they have every right to do so. I'm just saying without the concept of gender people wouldn't have that feeling to begin with. Only because society expects certain things from a certain gender do people feel like they don't fit. If we didn't have gender and simply accepted that some people have different characteristics from others without needing to label everything there wouldn't be any prejudice about gender and thus no dysphoria.
Caption this Meme 