Second preimage attack

[MBakhshi96]

As mentioned here Merkle trees are susceptible to the second preimage attack when a node can be presented as a leaf. To prevent this attack, OpenZeppelin typically uses double hashing for leaf values. However, in MerkleDistributor.sol, the leaf is constructed by hashing the value only once. Does this mean that the current implementation is not safe about these attacks? Is it assumed that they can't happen or are there other safeguards in the current contract?