Diamond is well known on r/Askhistorians for cherry-picking data and constructing his theory for a history that never happened. You can read about that here by /u/CommodoreCoCo and here by /u/anthropology_nerd.

For guns in Japan specifically, I talked about that here in response to Noel Perrin, which Diamond likely used as his source. In short, it is a bunch of bs. Yes, for safety (and for a short while, moral) reasons there were restrictions on the ownership and usage of guns. But there were plenty of guns both in government and civilian hands.