Who Said This?

[Gene C]

Getting closer. Benjamin Thomas was very familiar with our mystery writer's work.

Our person of interest has been mentioned many times on this forum, and has written many books and articles that are unrelated to Lincoln.

[Steve]

Paul Angle?

[Gene C]

A good guess, but he is not the author of the quote in question. 
Paul Angle was one of the first to express doubts about the Wilma Minor fake love letters between Lincoln and Ann Rutledge.

Our person of interest however was not so quick to regard the letters as fake. 
      "In a telegram and a letter, both dated October 19, xxxxxxx expressed reservations on a few specific points but came down firmly on the positive side. “My faith is strong,” xxxxxx declared, “that you have an amazing set of true Lincoln documents—the most extraordinary that have come to us in many, many years. The decisive influence of xxxxxxxx endorsement is revealed in the minutes of an Atlantic staff meeting held on October 19: “As xxxxxxx’s comments … were favorable, Mr. Sedgwick feels justified in going ahead with the magazine articles and with the book.”    (American Heritage Magazine)

However our person of interest retreated from this statement as more information came forth.