Being "really a woman, despite having a penis" is a contradiction, since by definition women don't have penises. That's why the TRAs have been trying to change the definition.

The act of believing it may not be falsifiable, that is, a man can say he believes he's a woman, and we can't prove he's bullshitting. But we can indeed disprove the factual accuracy of his belief by simply looking at his body, or failing that, giving him a chromosome test.