Did any Ecumenical Council find +John Cassian to be a heretic? Obviously, the Pope considered these theologies as theologoumenna and not dogma.
Both saints are venerated in the Orthodox and Catholic Churches.
From a Catholic perspective, St. John is considered to have erred in a semi-Pelagian direction. Some of his statements are not consistent with the canons of the Council of II Orange (a local council which is nevertheless considered to have dogmatic force, because of a later Papal endorsement of its decrees).
Harley is therefore wrong if he implies that St. John's semi-Pelagian teachings were, or are tolerated after II Orange (AD 529).
However, St. John died in AD 435. He was never condemned or censured in his lifetime, and so cannot be considered guilty of any formal heresy whatsoever, anymore than St. Thomas Aquinas can be considered guilty of formal heresy for rejecting the Immaculate Conception of Our Lady, which was not defined dogma for 600 years after his death.