When we read Tom Sawyer, we expect the words of Mark Twain to come through the characters. If we simply said that many unknown folks wrote Tom Sawyer, would the stories IMPACT be any less?
Granted, your "argument" is pure sophistry (to elevate it far higher than it deserves). Nobody would attribute "many unknown folks" to a work whose author is well known. It is when the authors of a work are much less well known that any questions arise. I am far too accustomed to creationists lying through their teeth; I would hope that you are not similarly inclined.
Granted, I read far more of Mark Twain's other works, such as
Roughing It,
The Innocents Abroad, and
Letters from the Earth. I believe that it was in
Huckleberry Finn that his most racist and pro-slavery statements were made. Was that Mark Twain "coming through the characters" to endorse racism and slavery? Or was he presenting a clear picture of someone steeped in racism and slavery expressing how he thought so that we "normals" could see it for the abomination that it actually is?
Answer Jar's question.