The Ecumenical Council of Ephesus was simply held in that city.
A great ecumenical council was held in Ephesus in 431, concerning whether the Virgin Mary might properly be called Theotokos, or bearer of God. The term had become popular in devotion and worship but was controversial. Many church leaders held that it was an appropriate title, reasoning that since Christ was both truly man and truly God, one could say Mary gave birth to God.
Nestorius, Bishop of Constantinople, and his party believed the term “Theotokos” threatened the humanity of Christ and denigrated the greatness of God, and suggested Mary be called “Christotokos,” bearer of Christ, instead. The council decided in favor of the Theotokos title, which has been used for Mary ever since. Nestorius was harrassed by mobs in Ephesus throughout the council, and exiled to Antioch afterwards.
Therefore anyone who denies the title “Theotokos” is a Nestorian, and is committing heresy. Plain and simple.
And fitting, given the apparent character and belief of those holding those beliefs, would you not say.