I have no idea if the Trump doctor actually wrote it, but the author makes interesting points backed-up by explicit reasons. It certainly reads like it's the voice of Donald Trump, who we know has posed as a PR person promoting Donald Trump -- and in the most effusive way possible. As is the case here praising Trump's health, as being the healthiest individual ever elected to the presidency. Of course, it's also possible that the doctor did write it all on his own. Or that he wrote it, but was told "This is what I want you to say."
I'm clearly not a doctor myself, so I can't say. I have been surrounded by such things my whole life, with my father having been a physician, as was my brother, and know they've said how doctors are always always pretty circumspect in making prognoses. And this is anything but. And I know too that when it comes to medical tests, you want the results to be "negative," since anything positive can be a sign of trouble. And this letter glows about how positive everything is. None of that is meaningful and substantive. But it's a personal perspective.
By the way, for a 70-year-old man to be in such near-perfect health, with pretty much nothing needing the slightest attention, is surprising on its own merits.
But who knows? It's an intriguing article, though, as the doctor-authors goes through the letter point by point. You can read it (and the original letter) here.
The bottom line for me, though, is that some people have earned our trust, and some haven't. Given his history of falsehood, going back far beyond this political campaign, but exacerbated by it, Donald Trump hasn't even come close to earning that trust.