They also mention that they're a robotics engineer, why should anyone care about that? Because most of the world isn't robotics engineers, so it's a useful thing to know for people who are also robotics engineers or are interested in robotics.
Humans connect via points of commonality, especially ones that aren't so widely shared.
I don't know, if we were on a LGBTQ forum, I'd probably ask the same question about the "robotics engineer" bit, whereas the trans bit would be normal to mention.
I do see your point about points of commonality, I'm just curious why it was just that one and not hobbies, etc then. I just don't think that being trans should be pointed out as being "niche", I'd rather it was just your gender and we forgot about it.
People attach different levels of importance to different parts of their identity. While some trans people may wish to only state their gender, or not even their gender, when introducing themselves to find community, for some other people being trans is an important part of their experience, and they would like to share it in order to make it easier for other trans or queer people to connect with them.
Humans connect via points of commonality, especially ones that aren't so widely shared.