I can assure you that submarine warfare is (and was) not at all “defensive in nature.” Submarines are very effective offensive platforms.
I’m guessing he meant Antisubmarine warfare was not attractive to the Japanese because it was perceived as defensive in nature – defending convoys.
The US had the same perception initially, and was slow to institute convoy.
However, as historian John Keegan explains in The Price of Admiralty, convoy procedure is effectively OFFENSIVE in nature.
The ocean is very big, and the odds of a submarine (pre-radar, pre-satellite) finding a ship are very small. Even a large convoy is not MUCH larger than a lone ship, and all the ships it includes are sailing at once, reducing the odds of a given sub finding anything – it gets one chance to find the passing convoy of 40 ships, not 40 chances spread over many days.
By leaving submarines no option but to attack the convoys, the ASW forces can in turn find the submarines – they’re where the convoys are – and attack THEM in turn. Early in WWII, separate groups of ASW forces performed “sweeps” looking for subs away from convoys. That was a failure, overall – the groups escorting convoys, however, found, and sank, plenty of subs, eventually forcing Doenitz, the German submarine leader, to withdraw his u-boats from the Atlantic battle in 1943.
Losses in the German u-boat service, proportinally, were higher than in any other service. Convoy techniques had lured the u-boats into a position from which ASW forces could engage them, aided by developing technology, and the results were devastating.
ASW work certainly could be offensive in nature. But the Japanese did indeed regard it as unglamorous, and they gave it far too little attention.
Sailboat