'John Lennon' and Paul McCartney actually did spend 24 April 1976 together as portrayed in the movie, but 'Yoko Ono' and Linda McCartney were also present. It was the last time they met. They got along so well that Paul returned the next day to see John. John would not let him in, saying, as he does in the movie, "You can't just be dropping by like it's the good old days."
When John Lennon offers Paul McCartney the chocolate, he says "Take this, brother. May it serve you well." This is a direct quote of a line Lennon spoke in the background to the song "Revolution 9" and in the scene, it is meant to be a joke between the two.