St. Leo I is explaining the clause - he is not treating these words as part of the clause. Don't turn to sophistry when trying to justify the unjustifiable.
We believe in one God, the Father Almighty, Maker of all things, both visible and invisible; and in one Lord Jesus Christ, the Son of God, begotten of God the Father, Only-begotten, that is, of the substance of the Father; God of God, Light of Light, true God of true God; begotten, not made; consubstantial with the Father; through whom all things were made, both those in heaven and those on earth, both visible and invisible; who for us men and for our salvation came down and took flesh, that is, was born perfectly of the holy ever-virgin Mary by the Holy Spirit, was made man, that is, He received perfect man, soul and body and mind and all that man is, except sin, not from the seed of man nor as is usual with men, but He reshaped flesh into Himself, into one holy unity; not in the way that He inspired the prophets, and both spoke and acted in them, but He was made Man perfectly; for "the Word was made flesh (John 1:14)," not undergoing change, nor converting His own divinity into humanity; -- joined together into the one holy perfection and divinity of Himself; -- for the Lord Jesus Christ is one and not two, the same God, the same Lord, the same King; and He suffered in the flesh, and rose again and ascended into heaven in the same body, and sits in glory on the right of the Father, about to come in the same body in glory to judge the living and the dead; whose kingdom will have no end; and we believe in the Holy Spirit, who spoke in the Law and proclaimed in the Prophets and descended at the Jordan, speaking in the Apostles and dwelling in the saints; thus do we believe in Him: that the Spirit is Holy, Spirit of God, Spirit perfect, Spirit Paraclete, increate, and is believed to proceed from the Father and to receive from the Son.We believe in one Catholic and Apostolic Church, and in one Baptism of repentance, and in the resurrection of the dead and the just judgement of souls and bodies, and in the kingdom of heaven, and in eternal life.
But those who say that there was a time when the Son or the Holy Spirit was not, or was made out of nothing or of another substance or essence, who say the Son of God or the Holy Spirit is liable to change or to becoming different, these people the Catholic and Apostolic Church, your Mother and ours, anathematizes; and again we anathematize those who do not confess the resurrection of the dead, and all heresies which are not consistent with this, the true faith.
St. Epiphanius of Salamis produced this modified Nicene Creed in 374 AD. Why didn't he know that it wasn't permitted to add orthodox truth to the Creed?
As for St. Leo, the Creed of Toledo was certainly a statement of faith (rather long, too - don't really want to type it all up here!) differing from the Nicene in words, although not in faith. The decree of Ephesus 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.
Now, the Creed of Toledo was certainly not word-for-word identical to that of Nicaea, although the same faith is there (and this is the sense of the similar canon of Chalcedon). The true interpretation of the Ephesine decree is that it prohibits creeds which have a different faith from that of Nicaea. Otherwise it would prohibit the Constantinopolitan Creed of 381 now wrongly known as the Nicene Creed, and also prohibit the Creed of Toledo approved by St. Leo (the widespread use of the Apostles' Creed in the West could also be cited - it didn't reach its final form until some time after Ephesus).