So monkeys prefer grapes to cucumber. Big deal.
A more interesting question is whether the monkey with the grapes share them with the other monkey, or does it act in a selfish way?
But that isn't the point. You could illustrate that with a much simpler experiment: give a monkey a choice between cucumber and grapes and see which it chooses.
The point of this experiment, surely, is that it shows how monkeys have in at least some sense a concept of
fairness. The monkeys are perfectly happy to eat cucumber. Nothing wrong with cucumber. What gets them angry is getting cucumber when someone else is getting grapes.
As for 'would they share the grapes' (at their own expense)? Well, this test doesn't test that, but I'm pretty sure I know what the answer would be.