While the Voice was speaking
Jesus was alone.
Jesus was alone.
(Luke 9:36)
Note: the Voice didn't stop before that Jesus was alone.
In Mark, instead, the Voice stopped and only after Jesus was found alone:
7 Then a cloud appeared and covered them, and a voice came from the cloud: “This is my Son, whom I love. Listen to him!”
8 Suddenly, when they looked around, they no longer saw anyone with them except Jesus.
This does a enormous difference.
In Marcion, Jesus has to be alone while the Voice is still speaking so that the reader can realize that only him is who has to be listened, in opposition to the demiurgical Moses and Elijiah. His solitude marks bluntly his power and superiority against the Demiurge.
To reduce a so powerful Jesus, "Mark" (editor) distinguished temporally the Voice and the rapid solitude of Jesus (by an interval of time), so that in this way Jesus's solitude becomes a sign of the his being rapidly abandoned by the previous divine possession and so a sign of the his being a mere man distinct from the spiritual Christ (separationism).
Evidently, a Jesus mere man was more convenient, for the banal judaizer Mark, against the excessive unique power of the marcionite Jesus.
In short words: the solitude of a mere man was useful to judaize the otherwise solitude of a god more powerful than the demiurge, the god of the Jews.