The cereal’s history and John Harvey Kellogg’s sexual ideology need to be distinguished. WK Kellogg’s company history dates its cereal origins to 1894, not 1878. That history does not establish that Corn Flakes were invented specifically to prevent masturbation.
John Harvey Kellogg’s own writing does document punitive attempts to suppress masturbation. In the 1881 edition of Plain Facts for Old and Young, he advocates circumcision without anesthetic as punishment for some boys. This historical connection to bodily control is the basis for the thematic association here, not proof of why a cereal box was placed in the film.
Primary source: Plain Facts for Old and Young, “Treatment for Self-Abuse”
See John Harvey Kellogg and Castration for the associated visual reading.