Why, according to the Catholic Church, does the Filioque not violate the Council of Ephesus?

Why, according to the Catholic Church, does the Filioque not violate the Council of Ephesus?

It is true that the Council of Ephesus forbids the writing of new creeds, but it does not forbid explaining the decrees in a clearer manner.

The Catholic Encyclopedia actually mentions this is it’s article on the Filioque!

Historical importance of the Filioque

It has been seen that the Creed of Constantinople at first declared only the Procession of the Holy Ghost from the Father; it was directed against the followers of Macedonius who denied the Procession of the Holy Spirit from the Father. In the East, the omission of Filioque did not lead to any misunderstanding. But conditions were different in Spain after the Goths had renounced Arianism and professed the Catholic faith in the Third Synod of Toledo, 589. It cannot be acertained who first added the Filioque to the Creed; but it appears to be certain that the Creed, with the addition of the Filioque, was first sung in the Spanish Church after the conversion of the Goths. In 796 the Patriarch of Aquileia justified and adopted the same addition at the Synod of Friaul, and in 809 the Council of Aachen appears to have approved of it.

The decrees of this last council were examined by Pope Leo III, who approved of the doctrine conveyed by the Filioque, but gave the advice to omit the expression in the Creed. The practice of adding the Filioque was retained in spite of the papal advice, and in the middle of the eleventh century it had gained a firm foothold in Rome itself. Scholars do not agree as to the exact time of its introduction into Rome, but most assign it to the reign of Benedict VIII (1014-15).

The Catholic doctrine was accepted by the Greek deputies who were present at the Second Council of Florence, in 1439, when the Creed was sung both in Greek and Latin, with the addition of the word Filioque. On each occasion it was hoped that the Patriarch of Constantinople and his subjects had abandoned the state of heresy and schism in which they had been living since the time of Photius, who about 870 found in the Filioque an excuse for throwing off all dependence on Rome. But however sincere the individual Greek bishops may have been, they failed to carry their people with them, and the breach between East and West continues to this day.

It is a matter for surprise that so abstract a subject as the doctrine of the double Procession of the Holy Ghost should have appealed to the imagination of the multitude. But their national feelings had been aroused by the desire of liberation from the rule of the ancient rival of Constantinople; the occasion of lawfully obtaining their desire appeared to present itself in the addition of Filioque to the Creed of Constantinople. Had not Rome overstepped her rights by disobeying the injunction of the Third Council, of Ephesus (431), and of the Fourth, of Chalcedon (451)?

It is true that these councils had forbidden to introduce another faith or another Creed, and had imposed the penalty of deposition on bishops and clerics, and of excommunication on monks and laymen for transgressing this law; but the councils had not forbidden to explain the same faith or to propose the same Creed in a clearer way. Besides, the conciliar decrees affected individual transgressors, as is plain from the sanction added; they did not bind the Church as a body. Finally, the Councils of Lyons and Florence did not require the Greeks to insert the Filioque into the Creed, but only to accept the Catholic doctrine of the double Procession of the Holy Ghost. - Filioque