While I absolutely agree with the overall premise - correct flatwork is the foundation for jumping - I would ask you to be careful with your word choice and be very precise in how you express it.
and I am always telling people that GP horses know minimally level 3 dressage,
Grand Prix jumpers may know and perform all the movements required at Third Level, but they don't perform them in the same frame or with the same goal. Is it important for you to be able to control a jumper's shoulders and move on three tracks at will? YES. Does a GP jumper execute a true canter pirouette or need to? Nope.
By similar logic, I could say that hunters perform Third Level dressage because they perform flying changes. Now, we know that's silly, because a hunter performs that flying change without the rhythm, cadence or suspension of the dressage horse. A hunter's flying change isn't recognizable as the Third Level horse's change, but technically, it's the same movement.
Now, the difference between a Third Level horse and a GP jumper is less than the difference between a Third level horse and a hunter, certainly - the dressage horse sustains collection, the GP jumper collects briefly, the hunter not at all, but a true Third Level horse looks a lot different performing third level movements than the jumper.
While I love dressage in and of itself, the emphasis on dressage and the dressage scores at the level levels of eventing is producing dressage-horses-that-jump, rather than brave and bold jumpers capable of an obedient dressage test.