Had fought Johnson as soon as he came out of prison, as Johnson wanted? Johnson weighed in for 225 against Willard, and then up to 242 in the following years. Fresh out of prisone, he dropped down to 212 pounds. What could a lean, early 40's Johnson of accomplished against Dempsey at that time if Johnson was motivated? Anything?
I doubt Harry Wills would have beat Dempsey, so a 40ish Jack Johnson wouldn't either. The Jack that destroyed Jim Jefferies could, but not the fresh out of prison one. Dempsey was a aggressive fighter who had a knack to plow through a fighter's defense, and Jack's would not have been as keen with his as he had been when he was the champ.
Johnson was decked and hurt by Ketchel, sucker punch or not. His ribs were also reportedly broken by Burns. Johnson himself pointed out that Dempsey had a style all his own. In the clinches, he'd discover why Damon Runyon called Dempsey the Manassa Mauler. Somehow, I doubt Lil' Artha' would be smiling in the ring with this Choctaw savage ripping into him.
Hence my emphasis on the word, "reportedly." The extent of the damage Dempsey did to Willard may have also been exaggerated. But Burns did have a punch, and he did go after Johnson's body with dogged determination. This is one of those allegations which might be interesting to research.