Israel could corporately be called God's son, and Davidic kings could receive filial language; therefore sonship terminology requires contextual interpretation.
Jesus distinguishes his heavenly Father's claim upon him even while speaking to Mary and Joseph.
The saying is Supporting rather than decisive evidence, but it establishes early that Jesus understands God as his Father in a distinctive and governing sense.
Mary and Joseph do not understand the saying.
Their incomprehension reinforces Luke's theme that Jesus' identity exceeds ordinary human expectations.