Yeah, it sounds like we're talking past eachother a bit then. I agree that you can't compare the four figures. That is not what the author is attempting to do, however.

The argument isn't that IF there are similarities between political rhetoric now and in 1800 THEN you may make a comparison between historical figures. It's that IF there are similarities between political rhetoric now and in 1800 THEN similar political problems may have existed in the year 1800 that we are facing today. The author is highlighting his premise, not making a character comparison.