No, Minn Kota has it "about" right. I believe their responder likely was referencing the fact that jon boats, canoes and kayaks all have rather distinctive and limiting theoretical hull speeds. That's likely what he or she was trying to convey.
So, imagine a stalled Volkswagen Beetle in a large flat, empty high school parking lot. You get lucky and two large 300 lbs. offensive linemen from the HS football team walk by and you convince one of them you need a push to about 100 yards away. So, one of the two agrees to it, feels little resistance, overcomes the inertia (objects not in motion resisting movement) and starts pushing as fast as he can, gets the car up to 15 mph. Not bad! So, the other 300 pounder is watching and decides to run up and push on the other side of the car's bumper. The Beetle now has two 300 pounders pushing on it. Yes, it "might" move a bit faster but . . . the max speed will ALWAYS be limited by the top end speed of the linemen. So, if the typical offensive linemen (let's say they are all the same) can run freely at 18 mph, you can have a dozen of them pushing on the back of a Beetle . . . and it will never exceed 18 mph. Actually, of course, something much less owing to the resistance of the car (friction, drag, etc.).
This is why a 55 "might" not increase speed all that much over a 30.
Where would added "thrust" help? If that Beetle is being pushed up a slope? Adding "thrust" will make a big difference. One 300 pounder might not be able to push it up a 45 degree slope, but two will do better, three even better and so on.
This is where more "thrust" comes in handy. A 55 will do better (go faster) than a 30 pushing into, say, a 20 mph headwind. It will also overcome inertia a bit better.
But, a big top end speed improvement in a weighted down jon boat? Nope. Not likely.
Brad