They're intersex, which means they have physical characteristics (could be just chromosones with "normal" genital presentation, or they could have a vagina with a mini penis clit, or some other configuration) that sit more in the middle of the fe/male sex spectrum. Since they say they're a "trans dude" and are taking T, I would presume their body naturally produces female hormones; but they feel more male, identify as such, and want to present to the wider world as being male.