If the men are able to exert twice the force on the plane as the elephant, it will have twice the acceleration.
True. But the question was ""Who can pull an airplane farther?".
All the elephant has to do is lean forward to negate 6-7 tons of the plane's weight by applying the mass of it's body to create that tension on teh rope that you mentioned. The men have to not only lean forward but then apply an additional 1.25 tons (minimum) of muscle strength just to match the elephant. (Neither would actually be apply a force equeal to their full mass but the ratio for each would be close enough to maintain the balance between them.)
While it's possible they could do that, how long could they continue to move the plane exerting the muscle energy required while the elephant is exerting no real additional energy? How long could they exert TWICE that amount of energy to achieve twice the the acceleration? (Since I am assuming it would be an identical plane for both, the decrease required by both would be the same once the plane starts moving..)