I found an explanation here. . This may seem kind of shaky compared to the logic we are used to, but there is a lot of overlap between the Pelagian and Nestorian heresies, they are of a family so to speak. So while Pelagianism wasn't mentioned directly, in essence the statement I believe is true. At the Quintisex council we see in Canon 1 an affirmation of the previous ecumenical councils, and in Canon 2 a list of secondary councils and documents that are to be considered authoritative and among these is the Council at Carthage although exactly which council is not mentioned.
I've never looked at this list in detail, it would be interesting to do.
Edited by Anna Stickles, 13 August 2016 - 01:03 PM.