They raise that possibility when discussing Experiment 3:
> Recall that in the induction trial, approximately equal numbers of infants chose each block—there was no systematic preference for any particular block. But what if all infants had their own idiosyncratic but consistent object preferences? ...
Then they sort of test that hypothesis in Experiment 4, by making the infants choose a block in the induction trial but then giving them the block they had not chosen – and even in this case the infants, in the next step of the trial, tend to reject the block they didn't get in the induction trial, which in this case was the block they had initially chosen. This needs to be combined with the previous findings, such as those from Experiment 2, in which the experimenters gave the infants a block, and Experiment 3, in which the infants' choice is blind (they din't know what was inside each box) — in these cases, the infants didn't show the same rejection tendency for the block they didn't get as in Experiments 1 and 4.
> Recall that in the induction trial, approximately equal numbers of infants chose each block—there was no systematic preference for any particular block. But what if all infants had their own idiosyncratic but consistent object preferences? ...
Then they sort of test that hypothesis in Experiment 4, by making the infants choose a block in the induction trial but then giving them the block they had not chosen – and even in this case the infants, in the next step of the trial, tend to reject the block they didn't get in the induction trial, which in this case was the block they had initially chosen. This needs to be combined with the previous findings, such as those from Experiment 2, in which the experimenters gave the infants a block, and Experiment 3, in which the infants' choice is blind (they din't know what was inside each box) — in these cases, the infants didn't show the same rejection tendency for the block they didn't get as in Experiments 1 and 4.