The textbook supposes an owner saying “This is my son” about an enslaved person older than himself. In this case, literal fatherhood is impossible because the other person is older. The question becomes whether an appropriate figurative legal use of the statement is available.
«هذا ابني»
This is my son.
The text attributes a finding of emancipation under the figurative reading to Abu Hanifa, but does not recognize that shift in this particular case according to the two companions. This is an illustrative ruling from historical legal doctrine on slavery, not a rule to transfer uncritically to modern family relationships.