That paper presents a good case that the layout was influenced by feedback from telegraph operators, but it doesn't really do much to refute the idea that changes were also being made for the purpose of preventing jams.

Obviously the claim that it was intended to slow typists down is unlikely. If the layout was rearranged in part to prevent jams, then it would have been rearranged so that nearby letters weren't hit in sequence. Without careful study of ergonomics, one wouldn't necessarily realize that that would slow a typist down. But it would probably prevent jams.

I also found some of the claims made in the paper very odd. They claim that because "SE" and "Z" are easy to mistake in Morse code, that those letters were placed near each other. How this would be helpful eludes me.