Newman’s maxim is not intended to be a “rule” that those Protestants versed in Church history “must” enter the Catholic Church. It is a general observation that Church history argues against Protestantism and that those Protestants who study history deeply many times realize that the Catholic Church is the true Church.
Ultimately, belief in the truth of Catholicism is a gift of faith given by God that must be accepted and acted upon by the recipient in order for the recipient to become Catholic. With Protestant historians, it may be that some, for whatever reason of divine Providence, were not given that gift; or that those who were given that gift did not accept it or act upon it for some reason. We must commend such people to God’s mercy, trusting that if these individuals follow him to the best of their ability according to the light of truth that they have, it may be possible for them to achieve salvation (cf. Lumen Gentium, 16).