All praise is due to Allah, and peace and blessings be upon the Messenger of Allah.
This statement is not part of the creed of Ahl al-Sunnah wa al-Jama'ah. Rather, it contradicts what the early generations (the Salaf) unanimously affirmed.
Ahl al-Sunnah believe that faith (iman) consists of speech of the tongue, belief of the heart, and actions of the limbs, and that all three are integral parts of the reality of faith. Deeds are not something external to faith.
Therefore, saying that deeds are merely a condition for the perfection of faith is contradictory, because a condition is external to the essence of a thing, whereas Ahl al-Sunnah regard deeds as part of the very essence of faith, not merely a condition for it.
Ahl al-Sunnah also believe that faith increases through obedience and decreases through disobedience. They do not separate deeds from faith, as some views that differ from the methodology of the Salaf do.
As for the general statement, "There is no disbelief except through belief (or conviction)," it is not correct. The Islamic texts indicate that disbelief (kufr) may occur through belief, speech, actions, or deliberate turning away from the truth, according to what is established by the Qur'an and the Sunnah, while taking into account the fulfillment of the necessary conditions and the absence of the recognized impediments.
The creed of Ahl al-Sunnah wa al-Jama'ah is that faith consists of speech, belief, and deeds, and that deeds are an essential part of faith itself, not merely a condition for its perfection. Likewise, disbelief is not limited to matters of belief alone; it may also occur through speech, actions, or other forms established by the Islamic texts.
And Allah knows best.