Fallacy, and unfair. You are tarring them just for agreeing with Taney.
Your snark doesn't address the merits.
Unfair? Go back and read the original question. The printed decision was heavily edited and changed by Taney, and the proof doesn't come from some testimonial by the other concurring justices, but rather, from the dissenters (who had cause to be concerned and take note) and from the physical evidence in the National Archives.
That Taney changed the Dred Scott decision, from the time he read it from the bench to the time it was finally published, is indisputable. Even Taney admitted he changed it. Aside from the fact that Taney wasn't supposed to change anything to begin with, under the rules of the Court, the question becomes whether the changes were substantial. Fehrenbacher presents proof that the changes added about 50% more text to the decision and in critical portions of Taney's arguments.