How to understand the last "joke" in the movie Confessions?

I haven't seen the original, but the last sentence in the movie means "joking", which means that both Rebirth and Bombing Mom are joking. There is a close-up in the movie in which Mr. Moriguchi defuses the bomb-this close-up clearly shows that there is no "mother bomb", because if there is a "mother bomb", there is absolutely no need to cut the line of the bomb and send it directly to Hideki's mother's office, let alone the close-up of cutting the line. In other words, human nature is complex, and it is not feasible to take one sentence as the only correspondence.