Great study. It is calling for a follow-up with neuronal recording, especially from mirror neurons in F5, to see whether neurons modulate their responses depending on observing intentional vs. accidental displays of reaching actions.
Minor comments : It would be interesting to dissociate the reward component of the task from the intention-direction component by placing the reward to the non targeted container.
1) Tamarins in the study were presented by the action in multiple trials and rewarded after each intentional grasp condition. This reward might have conditioned a preference toward the targeted container.
2) Moreover, the cue to the targeted container was more available from the reaching than from the hand flop trials. Therefore, reaching was a more reliable cue than a hand flop.
Interestingly, the main surprising findings, the existence of intentionality attribution and rational analysis in tamarins contradict with the major finding of the Herrmann-Tomasello study, which reported a significant advantage of human social cognition relative to nonhuman apes in theory-of-mind related tasks, like the one presented here.
It would be worth to rethink the possible adaptation of the Gergely experiment to monkeys and apes.
Conversely, it would be nice to replicate this study on 14 mo old babies.