"The five known manuscripts of the Gettysburg Address in Lincoln's hand differ in a number of details, and also differ from contemporary newspaper reprints of the speech."
en.wikipedia.org
The differences are almost all matters of punctuation -- e.g., some versions have commas or hyphens where others don't. There are also some trivial word differences, like whether it was "brought forth upon this continent" or just "on this continent," or whether the ground was "hallowed" or "consecrated." See the "
amalgamated version" in the Wikipedia article you linked to.
One version (the “Bliss copy,” the version we’ve almost always seen printed as “The” Gettysburg Address) has Lincoln’s signature. It is essentially identical (again, with differences in a few words plus punctuation) to the version printed in the Philadelphia Inquirer days after the speech.
So if we had five essentially identical copies of the Sermon on the Mount, one signed by Jesus, all essentially identical to a contemporary report‘s transcript of that sermon, and modern scholars
still argued the sermon was the product of a later editor, you would have good reason to be dismissive and to bring up the analogy with the Gettysburg Address.
Maybe a stronger analogy with the Sermon would be something like the "Melian Dialogue" in Thucydides'
History. From what I gather, scholars of the ancient world almost all say this was a "literary creation" as well, a "dialogue" whose words were never actually spoken by the people who were at that conference, but something Thucydides thought was a good representation of the kinds of argument which would be well suited to people in that position (the stronger side making demands of the weaker side). This is an entirely secular business, so presumably nobody is taking a stand one way or another out of any kind of religious partisanship for or against the reliability of Thucydides; sometimes scholars just think they have good reasons for skepticism about whether things happened the way the author said they happened.
Now it's possible that some scholars go out of their way to cast doubt on religious texts by using standards or criteria that they
don't use with secular texts. And it's possible that many are too skeptical of both secular
and religious texts. But that's something that would have to be argued for.