Hi! Alas, they are incorrect on such a need. Not just will it interfere with the car's trailer sway control system, but it loads the frame differently than Subaru intends. Subaru made the car, they know best, lol.
And, that's a tiny amount of weight compared to what I towed with some very decent gusts in the 35-40mph range.
As for the Trailer Stability Assist, the Ascent uses the Traction Control/ABS system to brake certain wheels to mitigate sway conditions. Here's a really poor video showing how a tail weight on the flat trailer did not allow the trailer to yank the car out of control back and forth. Watch it on a phone or something you can put into portrait mode, and you will just barely notice that the car squats a corner here and there (and appears to yank downwards on that corner) to mitigate the pendulum like motion of sway that a rear weighted trailer like that would cause when whipping around turns.