Yes, that's true. In a Hunter under saddle class, a dressage horse can and does do very well IF it's shown in a training level frame or allowed to move in huntery frame. And yes, it would be easier to show a dressage horse in a hunter under saddle class and pin well that it would be to take a hunter and show training level dressage and do well - you'd have to ask the hunter to carry more frame and move through its back; though I think there are some horses who could make the switch quite well.
It's still true, though, that at a rated show and a rated division, the under saddle portion is 25% of the division (used to be 1/3, back in the day, but it was decided to lessen the emphasis on the flat and increase the importance of over fences.) The dressage horse would require some signifigant schooling to compete in the over fences portion.
None of which changes the fact that the original post in this thread starts with a faulty premise, an apples to oranges comparison, and an offensive, discipline-centric, chauvinistic point of view.