People making claims like that generally no nothing of the procedure by which mental health issues are diagnosed - which is to say, mental health treatment is about addressing issues which are negatively affecting someone's quality of life, until they are not. This is the same reason you can't just go through the DSM V and ascribe mental health conditions to people on TV or in the media or whatnot - it doesn't work that way.
People who hear voices don't have a problem because they hear voices, they have a problem because in general schizophrenia presents those voices as hostile and aggressive towards the person. People who hear voices that are positive or helpful (which is not uncommon, though it seems to be less common in Western culture socialized people) don't have a problem, probably don't even need treatment if they do not cause any distress.
That trans-people live happier lives after transitioning means that the treatment they receive (hormones,gender reassignment surgery, remembering that all of those are quite optional) is completely adequate.