Although it obviously does not result from this letter, the presbyter Sixtus is the future Pope Sixtus III (432-440). He is mildly reproached by Augustine for his earlier writings that may have been considered pro-Pelagian. The rest of the letter is a long anti-Pelagian treatise. Augustine mentions it in "The Gift of Perseverance", 55 (PL 45, 1027). See also the previous letter of Augustine to Sixtus .
As far as "the declaring of the Pelagians to be anathema" by Sixtus, it certainly does not refer to any formal decision or action by the presbyter, but only to his public declaration of considering the Pelagian views as heretical.