Was King Arthur a Pelagian?

Christianity Today's Christian History and Biography section makes a point about an underlying anachronism of the 2004 Antoine Fuqua King Arthur movie. Arthur is shown in the movie wearing a Pelagian medallion signalling his belief in what was a contemporary heresy that rankled the likes of Saints Augustine and Jerome.

"A deleted scene shows Arthur meeting Pelagius. The date is all wrong — Pelagius was born around 354...."

Christianity Today - Was King Arthur really a Pelagian?
Pelagius
Bede's Ecclesiastical History - Sections on Pelagius
The Early Church
In response to attacks from Augustine and Jerome, Pelagius (a monk and theologian) wrote De libero arbitrio (On Free Will) in 416 and was excommunicated by Pope Innocent I the following year. After Zosimus succeeded Innocent, he confirmed the condemnation of Pelagius.