No doubt Johnson was a much better boxer than Jeffries, but that is no reason to count Jeffries out. He had been thrown in against skilled boxers, within months not years of starting professional boxing full time. It is not far fetched to think, that he had the style and physicality, to walk Johnson down. This fight is up for grabs.
Johnson’s durability is a question for me too. Jeffries may or may not maul him out of the fight entirely. I see them as on a similar level (Jeffries probably under-rated, Johnson, most definitely over), just very different stylistically.