Probably because that (BP giving Churchill advance notice) never actually happened.
https://www.gchq.gov.uk/information/did-signals-intelligence-predict-attack-pearl-harbor
The idea of a subversive conspiracy to undermine social and political order first appeared in the wake of the French Revolution when a French Jesuit wrote about Jacobinism, ascribing the outbreak of the Revolution and the Terror to the machinations of Enlightenment thinkers and secret societies...