First of all, bridget is a girl, that's the cannon, even if it was made in a poor way (which it wasn't) she'd still wouldn't be a boy.
Then, you're oversimplifying the story. The idea that they could have left the story as it was is right, but they didn't have to because characters are fluid, just as people, they evolve through their whole existence, and thus, ggst Bridget's story doesn't contradict her earlier story. Also, the femboy to trans woman is something very common, people just decide to be a femenine man until they realize that is not enough, like what happened to me. Anyways, ik you don't care and just haye trans people getting any kind of rep, just posted this here in case someone saw this and actually had an interest in using their brain and listening to someone with relevant experience to this.