The author of the text I've posted says:
Pablo Fanque, Mr. Kite and the Hendersons were never more than colourful names to John Lennon, but they were real stars in the circus world.
Of course that John used the names and words from that old poster to play with and make a song - he was a songwriter.
Did John had to go to investigate the whole history of the persons mentioned in the song? Maybe the Beatles should give a task to their press staff to do a research.
But, John's song did a lot more - a lot of people know and talk about those people from the song!
It is like with that beautiful tune 'Soldier of Love' (from the BBC album) - some people say that it would be forgotten if the Beatles didn't make a cover of it.