Publicly revealing Yamato around 1940 could have put political pressure on British and American battleship programs already under way. American politicians might question ships much smaller than the Japanese design and demand 18-inch guns despite the Navy's earlier decision against them. Britain could face pressure to complete the Lions instead of accepting Vanguard's 15-inch armament. The proposed benefit to Japan is disruption of procurement decisions. T-class submarines are offered as a more practical British response than matching Yamato gun for gun.
Notable Point · 3:38:32-3:39:58
How revealing Yamato in 1940 might disrupt Allied shipbuilding
An announcement of Yamato's capabilities could have provoked political demands for larger Allied battleships after existing construction plans were committed.