What is the meaning of "from a man he named" in the isnad, and why was the author not mentioned even though he is the second in line from his side, and is such an isnad considered valid, knowing that the narrator from the unknown person is Duhaym, one of the great scholars of al-Jarh wa al-Ta'dil?

His saying in the chain of narration, "from a man he named," could mean that the narrator forgot his name and thus left it ambiguous (which is more probable), or he left it ambiguous because the unnamed person does not meet his criteria for authenticity. This chain of narration cannot be used as proof, because the narration of an ambiguous person is rejected, even if he left him ambiguous while still vouching for his trustworthiness.