5:18 Is Shirk Or Tribalism Being Rebuked Or Both?
This is not the description of their shirk, but of their vanity. They never had this idea they were real children of God. Referring to their human origin is not telling them that they are the same like others, but humility of their origin that they should not thing of themselves too great or sacred, and that, they do not have any immunity from the punishment.
But then why mention a bnaa when their vanity is captured in ahibbaa? What is the purpose of mentioning it if Allah himself in the Old Testament calls them their father them his children (in a metaphorical sense)