Well, I would guess that most of the people in F are on full F fares, because there are no discounted buckets.
In Business class, it's probably a relatively small handful on J, but many more on D/I fares. The only time to purchase a J fare is when the D and I buckets are sold out. In fact, the lowest available refundable Biz fare is the Y3CUPMZ which actually books into "I" inventory, and there's no advance purchase. And you're already talking about $3,500 r/t, so if I is sold out and you have to purchase a more expensive J/D fare, chances are you'll probably find a cheaper fare on UA or DL or CO, making it very difficult to justify sticking with AA.

Of course, the above excludes the impact of corporate negotiated fares, which for large businesses, may always book in J inventory at sharply reduced rates. So those folks may be best positioned to benefit from the promotion, even if they can't fly Delta anyway because of company policy. Kind of perverse.