There are numerous signs of her authorship:

Here’s the line — in paragraph 5 — that prompted my exclamation: “A monster who recognized my husband as an inhuman political machine.” A native speaker of English would not have used the word “recognized” like that. I’m sure many people are mocking her for that now — yes, even monsters can see that Trump is an inhuman political machine — but to me, it means she wrote it herself.

Later, she repeats the idea of Trump as a “political machine,” but the idea is that he is human: He has a “human side” — “core facets” that include “laughter, love of music, and inspiration” — but these things are “buried below the political machine.” That’s an individual, perhaps awkward, way to say that the would-be assassin saw only the political surface and not the real human being. 
Some other signs of her personal voice: 
“the fabric of our gentle nation is tattered” 
“attempted to ring out” (should that have been “wring out”?)
“Political concepts are simple when compared to us, human beings”
“American politics are only one vehicle that can uplift our communities”
“while we are here, in this earthly realm”
“Dawn is here again”
“love transcends”
“The winds of change have arrived”

AND: There’s also, near the beginning of the letter: “When I watched that violent bullet strike my husband, Donald, I realized my life, and Barron’s life, were on the brink of devastating change.” I don’t think some ghostwriter would have come up with the idea of putting herself first and her son second.

That made me think of the old Lenny Bruce line about Jackie Kennedy, which was much more harsh but the same idea of thinking of yourself first: Jackie crawling out of the convertible was “hauling ass to save her ass.”

CORRECTION: I had “nature” for “nation” in “the fabric of our gentle nation is tattered.” “Nature” was more idiosyncratic, but she had “nation.” Sorry. I do think “gentle nation” is unusual along with picturing it as tattered fabric.