Biblical prophets can be sent by God, but they are not ordinarily described as personally descending from heaven.
Exodus 16 remains the immediate manna background.
The contrast between known earthly parentage and Jesus' claim to have descended from heaven makes personal pre-existence explicit.
This is strong evidence that John's Jesus understands his life as a heavenly descent, not merely an earthly prophetic vocation.
The hearers grumble because they know Joseph and Jesus' mother.