Because of Jack’s relationship with Bert he apparently knows everything about Mary Poppins.
Mary Poppins Returns is currently filming in London with Lin-Manuel Miranda and Emily Blunt.
Miranda has said that part of his role in the film is to inform the other characters about how cool stuff happens when Mary Poppins is around.
Miranda plays Jack, a lamplighter who was, according to Miranda, an apprentice of sorts to Bert, the character played by Dick Van Dyke in the original movie.