It is true that the Council of Ephesus (431) prohibited the making of new creeds. It stated,
It is not permitted to produce or write or compose any other creed except the one which was defined by the holy Fathers who were gathered together in the Holy Spirit at Nicaea. Any who dare to compose or bring forth or produce another creed for the benefit of those who wish to turn from Hellenism or Judaism or some other heresy to the knowledge of the truth, if they are bishops or clerics they should be deprived of their respective charges, and if they are laymen they are to be anathematized. (Definition of the Faith at Nicaea)
Edicts of an ecumenical council are binding on Christians, but they are not binding on another ecumenical council unless they are pronouncing a matter of faith or morals. Later ecumenical councils can revise or modify disciplinary policies of their predecessors. Since the prohibition on making a new creed was a disciplinary matter, it could be changed by later ecumenical councils.
At the ecumenical Council of Florence (1438-45), it was changed, and the council ruled that the words "and the Son" had been validly added to the Creed. The Eastern Orthodox originally accepted the authority of the Council of Florence, but later rejected it.
Note that Ephesus referred to the creed as composed by the Fathers at Nicaea (325), not as modified at Constantinople. This is significant because the final portion of the Nicene Creed, which deals with the Holy Spirit and contains the filioque clause, was not composed until the First Council of Constantinople (381). If the prohibition of Ephesus undermined the modern Catholic creed, it undermines the Eastern Orthodox creed no less, since the Eastern Orthodox version includes the material on the Holy Spirit as written at Constantinople I. It is inconsistent for the Eastern Orthodox to cite Ephesus about the filioque clause when all of the material on the Holy Spirit was added to the creed that was formulated at Nicaea.
Ephesus's prohibition of making a new creed in addition to the Nicene prompted questions about the status of the material added by Constantinople I. How this material was to be regarded was settled at the ecumenical Council of Chalcedon (451), which stated,
Therefore this sacred and great and universal synod . . . decrees that the creed of the 318 fathers is, above all else, to remain inviolate. And because of those who oppose the Holy Spirit, it ratifies the teaching about the being of the Holy Spirit handed down by the 150 saintly fathers who met some time later in the imperial city--the teaching they made known to all, not introducing anything left out by their predecessors, but clarifying their ideas about the Holy Spirit. (Definition of the Faith).
According to Chalcedon, it was permissible for the Fathers of Constantinople I to include the material on the Holy Spirit in the Creed of Nicaea; they were not adding substance but clarifying what was already there. Yet if this option of making clarifying notations to the creed was permissible for them, it would be permissible for others also. Thus the Council of Florence could add "filioque" legitimately as a clarification of the manner of the Spirit's procession.