I think the point is that you can't run a motor off the solar panel itself -- it has to be off a battery. And whether you have enough battery power to get you where you want to go is something that you can calculate, if you know the amp draw of the motor and the available amp capacity of the battery or batteries available to you. Of course, whether it pushes you fast enough to suit your purposes is a whole other story.
This doesn't account for the impact of prop design, which motor boaters will tell you is critical to getting maximum performance from a given hull. When I worked commercial trips on the Grand Canyon a few years ago and we got new Hondas, the first thing we did was throw away the stock prop and put on one that was more suitable to 37' baloney boats. So it's possible one trolling motor prop might be better suited to pushing a raft than another and that might account for the performance differences mentioned here.
As to horsepower, I've used a five horse Honda (weighs 65 pounds) on lots of rivers and lakes with my 16' cat. It will get you there, but not very quickly -- 8-10 mph would be tops, depending on wind and wave conditions. That motor does, however, get about 25 mpg on an average trip with the cat, which is incredible fuel economy. It gets about 15 mpg when I put it on my 14' NRS SB -- a shorter boat with more frictional surface on the bottom and more frontal area for the wind to work on.