ANY trailer will sway given the right circumstances! To say that a trailer that is set up correctly WILL NOT sway is incorrect. Also, most serious "sway" events start out with a "wiggle"! I agree that the wiggle from a passing semi or gust of wind is usually not serious enough to start a sway event but, under the right circumstances, it could be.
See my post made a couple of years ago
here.If a person were going downhill, braking for a curve, and being passed by a fast moving semi or bus, or get hit with a wind gust, then that wiggle could very easily turn into a serious event. I think all travel trailers should use some kind of sway control regardless of whether they have the hitch setup correctly, the trailer loaded correctly, the tires aired up correctly, the axles aligned correctly etc. etc. etc.
Barney