This is not true: person B owns the copyright to his/her contributions. As soon as person B added code, the copyright of the project no longer belonged solely t

This is not true: person B owns the copyright to his/her contributions. As soon as person B added code, the copyright of the project no longer belonged solely to person A. And that's not a matter of opinion but legal fact. It would however be possible for person A to remove all contributions of person B (and possibly rewrite the same functionality).