I'm not a Ken Burns fan (to put it mildly), but the photos and films he excavates are second to none.
From what little I could tell about the films, fighters of that era used a completely different style than today's boxers: Straight up, very little footwork, they carried their gloves a lot lower than nowadays, and most of them looked slow.
Maybe the lack of footwork was because fights lasted a lot longer then, and they couldn't dance around for 20-30 or more rounds.
Johnson and some of the others were physical speciments. Nevertheless, I question whether any of them would last very long in the modern ring using that style. (Let them train and get used to modern methods, and maybe it would be another story.)
I did not see the Burns documentary but I used to be a boxing fan years ago so I know a little about this.
You are right about Johnson. His style was very forward looking. Jeffries was old school, even primative (the same can be said for Corbett and Fitzsimmons).