I think he's referring to when one of the Zerocoin developers mentioned that a backdoor could be added for government surveillance (ie, they would know what your redemption code is or something like that). They probably said that so they wouldn't be arrested for violating AML laws. Their FAQ says it's open source so nobody can add a backdoor without everyone noticing. http://zerocoin.org/q_and_a#could-you-put-a-backdoor-in-it Also, since it's open source, if they were forced to add a backdoor, others could fork libzerocoin and remove it.