When Jesus says that he has come down from heaven, his hearers object: 'Is not this Jesus, the son of Joseph, whose father and mother we know?' Their response reflects the ancient importance of family identity and place of origin.
They believe they know where Jesus comes from because they know his household.
In the ancient Mediterranean world, people were commonly identified by parentage, kinship, hometown, or other social relationships. Such identifiers located a person within a recognizable community. Calling Jesus 'the son of Joseph' therefore expresses ordinary social knowledge rather than an attempt to answer John's theological claims about incarnation.
The Gospel has already told readers that Jesus' ultimate origin cannot be reduced to that social identity.
The cultural setting sharpens John's irony. The crowd's information is not meaningless: Jesus truly has a recognizable human history and family setting.
Their mistake is treating that knowledge as exhaustive. John's prologue has prepared the reader to see that the one known in ordinary human terms is also the Word who came from God.