Divorce for the Noahide Children – Jerusalemite

Hello Rabbi,

The Jerusalemite at the beginning of Kiddushin discusses divorce among the descendants of Noah and above, two possibilities: there is no divorce among them at all (it does not matter at this point whether the intention is to not allow the marriage, or just that there is no divorce in the certificate), or there is a mutual divorce. As evidence for the system in which they do not have divorce, the following is presented:

“Rabbi Haya gave me a case where a Gentile who divorced his wife, who went and married another and divorced her, and later both of them converted, I do not read about her, her first husband, who sent her away, will not be able to return to take her, and let me say that such a case came before Rabbi and was valid.”

In other words, since there is no real divorce, or at least not in the document, then they are not prohibited on the part of the one who returns his divorced wife after she marries someone else. But it is not clear to me: seemingly there is no need for this, after all, a Jew who converted as a minor was born of his blood. So what is the present Amina that would be prohibited?