Due to their political bent, they formed a conclusion that transgender people are not mentally ill and then came up with reasons to support their conclusions.
But let's use some common sense and basic reasoning and use Occam's Razor, elements sorely lacking among the left. I'm 60. If I insisted I was 26 and really believed it, I'm mentally ill. If I insisted I'm a cat and really believed it, I'm mentally ill. If I'm a man and say I'm a woman, I'm mentally ill.
This is really simple to figure out. You're welcome.