The embarrassment is that Jesus is already dead - with John as 100% witness of this death - even before that the entire story started, on the earth
Hence, if John is described as not a real witness of this death, since the latter is transformed as a mere earthly baptism, then the story can start - freely and undisturbed - on earth
But Celsus says us that just John witnessed the descent of Jesus. And Celsus says us indirectly where
John witnessed Jesus: in Sheol
When you were bathing, says the Jew, beside John, you say that what had the appearance of a bird from the air alighted upon you. And then this same Jew of his, continuing his interrogations, asks, What credible witness beheld this appearance? Or who heard a voice from heaven declaring you to be the Son of God? What proof is there of it, save your own assertion, and the statement of another of those individuals who have been punished along with you?
Just the phrase:
who have been punished along with you
...implies that John was already dead - hence, he was in Sheol
- when he witnessed first
the revelation of Jesus as a divine being, by the "baptism".
Celsus's criticism is: how can a soul in Sheol as John be a credible witness of the divine nature of Jesus?
Clearly, Celsus's Jew
wanted a living
witness, not the witness of a soul in Sheol.