Though it is true Sir Elton copied the name of his bandmate, Elton Dean, to come up with his first name.
He told Rowan Atkinson in 1991: “I used to be in a band, and I wanted to become a singer in my own right, and I wanted to choose a name.
“And the saxophone player in the band was called Elton, so I chose that name.”
However, his last name is not believed to have come from John Lennon, and the moment in the movie is more likely a tribute to the Beatles star.
In fact, the band which Sir Elton began his life, Bluesology, was the support act for performer Long John Baldry, which is believed to be where his surname came from.
source: express.co.uk